BACKGROUND
the yeast yarrowia lipolytica is a hemiascomycete and represents a homogeneous phylogenetic group with physiological and ecological diversity  <cit> .
it is a non-conventional yeast, often used in research and isdistantly related to candida glabrata, kluyveromyces lactis and debaryomyces hansenii.
strains of y. lipolytica can produce significant amounts of intra- or extra-cellular metabolites including vitamins, lipases, storage lipids, citric acid and pyruvic acid and can be used for biodegradation of various wastes   <cit> .
the  <dig> -dihydroxy phenyl l-alanine  is a drug used for parkinson's disease, and is capable of changing the enzymes of energy metabolism of myocardium following neurogenic injury.
the process of bioconversion of l-tyrosine to l-dopa in microorganisms is generally slow, but is accelerated by a small amount of l-dopa in the broth  <cit> .
l-dopa has also been produced with erwinia herbicola cells carrying a mutant transcriptional regulator tyrr from pyrocatechol and dl-serine  <cit> .
it can also be produced using l-tyrosine as a substrate, tyrosinase as a biocatalyst and l-ascorbate as the reducing agent  <cit> .
the general reaction is:
tyrosinases  belong to a larger group of type- <dig> copper proteins, which include catecholoxidases and oxygen-carrier haemocyanins  <cit> .
tyrosinases are involved in the melanin pathway and are responsible for the first steps of melanin synthesis from l-tyrosine, leading to the formation of l-dopaquinone and l-dopachrome  <cit> .
tyrosinases catalyse the o-hydroxylation of monophenols  and the ensuing oxidation of molecular oxygen.
subsequently, the o-quinones undergo non-enzymatic reactions with various nucleophiles, producing intermediates  <cit> .
the immobilization of tyrosinases on solid supports can increase enzyme stability  <cit> , protect tyrosinase from inactivation by reaction with quinones,   <cit> , improve thermal stability of fungal tyrosinases  <cit> , and increase activity in comparison to soluble enzymes  <cit> .
diatomite  is a naturally occurring, soft, chalk-like sedimentary rock that is easily crumbled into a fine, off-white powder which has k+ in the interlayer.
this powder has an abrasive feeling similar to pumice and is light-weight due to its porosity.
by adding diatomite into the reaction, an increased substrate uptake and enzyme production rate with concomitant l-dopa production could result.
we have previously reported the effect of cresoquinone and vermiculite on the microbial transformation of l-tyrosine to l-dopa by aspergillus oryzae  <cit> .
in the present study, different concentrations of diatomite were added into the reaction mixture to achieve a high performance transformation of l-tyrosine to l-dopa.
a. oryzae is an organism typically used for l-dopa production.
the easy handling, rapid growth rate and environmentally friendly nature of alternative yeasts such as y. lipolytica have created an interest in their use for fermentation.
because tyrosinases are intracellular enzymes, pre-grown cells harvested from fermented broth were used for the microbiological transformation of l-tyrosine to l-dopa.
RESULTS
the production of l-dopa is largely dependent on the addition of specific additives and minerals to the reaction mixture.
the inductive effect of diatomite on the transformation of l-tyrosine to l-dopa by yarrowia lipolytica nrrl- <dig> was investigated .
the concentration of diatomite added at the start of biochemical reaction ranged from  <dig> – <dig>  mg/ml, along with  <dig>  mg/ml l-tyrosine.
a biomass concentration of  <dig>  mg/ml was used as a source of intracellular enzyme tyrosinase in a  <dig> min reaction.
the highest production of l-dopa  was observed with  <dig>  mg/ml diatomite.
l-dopa production fell while substrate consumption continued to rise, probably due to catecholase activity causing l-dopa to be used for quinone production, since ascorbic acid  was not being replaced in the system.
in some enzyme systems, disaccharides or higher molecular weight substrates have been found to be the best supporters of intracellular enzymes  <cit> .
it was hypothesized that tyrosinase, a constitutive enzyme, was altered with respect to production of l-dopa in the presence of added diatomite.
the effects of delayed diatomite addition  into the y. lipolytica nrrl- <dig> reaction were also investigated .
reactions were performed aerobically with  <dig>  mg/ml cell biomass and  <dig>  mg/ml l-tyrosine for  <dig> min.
production of l-dopa increased from  <dig> to  <dig> min after the addition of diatomite; a significant decrease of l-dopa  was noticed 20– <dig> min after the addition.
maximum l-dopa  was obtained  <dig> min after the addition of diatomite into the reaction mixture, with concomitant tyrosine consumption of  <dig>  mg/ml, a 35% increase when compared to the control which is highly significant .
the l-tyrosine substrate has binding affinity with diatomite, which induces tyrosinase secretion, improves its availability and ultimately leads to an increased l-dopa production rate  <cit> .
in our experiment, the addition of diatomite  <dig> min after reaction commencement was identified as optimal, increasing production of l-dopa, substrate utilization and time of reaction.
however, l-dopa production dropped  when diatomite was added  <dig> min after the start of reaction, probably due to conversion of unstable l-dopa to dopamine, melanin and other pigmented products  <cit>  after a reduced availability of the enzyme.
the consumption of l-tyrosine, however, continued to increase despite the time of diatomite addition.
the tyrosinase active center is comprised of dinuclear copper, coordinated with histidine residues, chelating substances or substances that are associated with this metal  which are irreversible inhibitors and/or inactivators of this enzyme  <cit> .
the addition of diatomaceous earth may remove these inhibitors and/or inactivators by active absorption.
the absorption of inhibitors increased the enzyme activity of tyrosinases, β-carboxylases and tyrosine hydroxylases which was important for the catabolism of l-tyrosine to l-dopa under controlled conditions.
our data are both substantiated  <cit>  and in contrast to previous reports  <cit>  in which the production of l-dopa was achieved in minimal medium without additive supplementation .
previous research efforts to produce l-dopa by the addition of  <dig>  μg vermiculite during the reaction obtained  <dig> – <dig>  mg/ml of the desired product  <cit> .
the time course of l-dopa production and l-tyrosine consumption was carried out at different incubation periods  using a hotplate with magnetic stirrers .
the control gave a maximum of  <dig>  mg/ml l-dopa with  <dig>  mg/ml consumption of l-tyrosine.
the maximum conversion rate  was obtained with  <dig>  mg/ml diatomite added  <dig> min after the start of reaction, producing a 72% higher yield of l-dopa compared to the control.
the l-dopa production from this time course differed significantly  with the results at all other incubation periods.
it is clear that up to  <dig> min, cresolase activity predominated and, given the non-replacement of ascorbic acid, the overriding activity was catecholase, which consumed the l-tyrosine substrate without a corresponding production of l-dopa.
after 40– <dig> min of incubation, the production of l-dopa and the consumption of l-tyrosine decreased gradually in the control and test reactions.
this reduction might be because the l-dopa and residual l-tyrosine were changed into other metabolites such as dopamine, melanin and eventually melanosine.
another study  <cit>  achieved  <dig>  mg/ml of l-dopa,  <dig> min after the biochemical reaction.
the present finding of  <dig>  mg/ml l-dopa after  <dig> min of incubation is a major improvement.
in the present study, dopamine and melanin were also produced, but their highest production was  <dig>  and  <dig>  mg/ml/h.
conversion of l-tyrosine to l-dopa is an enzyme catalyzed reaction.
figure  <dig> shows the effect of the addition of different concentrations of drenched cell biomass  on the production of l-dopa from l-tyrosine in the reaction mixture.
the best results  were obtained using  <dig>  mg/ml wet weight yeast cells, leading to  <dig> fold higher productivity when compared to the control .
at this concentration , most of the added tyrosine was converted to l-dopa as indicated by the small amount of residual substrate , which is highly significant .
in the present investigation, the increased cell biomass enhanced enzymatic activity .
however, increasing the cellular concentration beyond the optimal led to a sharp decrease in activity, probably due to increased cell concentration  and the maintenance of a constant concentration of an inhibitor of catecholase activity .
this product is the substrate for the second reaction catalyzed by this enzyme  which leads to the formation of quinones from l-dopa.
only an excessive amount of ascorbic acid continually replaced throughout the reaction might stop this second activity from taking place, leading to the formation of quinones that are also suicide inactivators of this enzyme.
previous research  <cit>  pointed out that tyrosinase activity is directly related to the concentration of cells or mycelia in the reaction mixture in slightly acidic to neutral reaction conditions.
copper atoms found at the active site of tyrosinase are an essential requirement for catalytic activity.
agents such as carbon monoxide or toxins indirectly inhibit tyrosinase activity by chelating copper and abrogating its ability to bind oxygen.
previous research  <cit>  has shown that tyrosine phenol lyase  is only synthesized under l-tyrosine-induced conditions.
the addition of l-tyrosine to the medium was found unavoidable when preparing cells , but severely impeded preparation of pure l-dopa  <cit> .
a comparison of production parameters for the effect of diatomite addition on bioconversion of l-tyrosine to l-dopa is shown in table  <dig>  an overall  <dig>  fold increase in l-dopa production  was achieved at the optimal level of added diatomite when compared to the control.
the optimal ph of the control reaction without added diatomite was  <dig> , however, the test reaction with added diatomite was proficient at a ph range of  <dig> – <dig> , indicating the enzyme remained active despite the change in reaction ph.
the yp/s value  was significantly improved over the control.
maximum substrate consumption  in terms of volumetric rate was marginally different during bioconversion between the control and test reactions, indicating maximum enzyme activity at this level of diatomite addition.
the increase of qs  with diatomite addition was highly significant .
in the present study, the optimal values of all kinetic parameters  were several-fold improved over those reported from aspergillus or cellulomonas spp.
<cit> .
*kinetic parameters: yp/s = mg l-dopa produced/mg substrate consumed, qs = mg substrate consumed/ml/h, qs = mg substrate consumed/mg cells/h.
** <dig>  mg/ml diatomite added  <dig> min after the start of reaction.
$acetate buffer
*$<p> is for significance level  on the basis of probability.
hs denotes that the values are highly significant.
CONCLUSIONS
in the present studies, yarrowia lipolytica strain nrrl- <dig> was exploited for l-dopa production.
the addition of  <dig>  mg/ml diatomite  markedly improved the microbiological transformation of l-tyrosine to l-dopa.
diatomite addition  <dig> min after the start of reaction produced a 35% higher substrate conversion rate compared to the control .
a biomass concentration of  <dig>  mg/ml and reaction time of  <dig> min were also optimized.
because production of l-dopa is a high cost, low yield process, scaled up studies are a pre-requisite for commercial exploitation.

 the yeast yarrowia lipolytica is a hemiascomycete and represents a homogeneous phylogenetic group with physiological and ecological diversity. it is a non-conventional yeast, often used in research and isdistantly related to candida glabrata, kluyveromyces lactis and debaryomyces hansenii. strains of y. lipolytica can produce significant amounts of intra- or extra-cellular metabolites including vitamins, lipases, storage lipids, citric acid and pyruvic acid and can be used for biodegradation of various wastes. the -dihydroxy phenyl l-alanine is a drug used for parkinson's disease, and is capable of changing the enzymes of energy metabolism of myocardium following neurogenic injury. the process of bioconversion of l-tyrosine to l-dopa in microorganisms is generally slow, but is accelerated by a small amount of l-dopa in the broth. l-dopa has also been produced with erwinia herbicola cells carrying a mutant transcriptional regulator tyrr from pyrocatechol and dl-serine. it can also be produced using l-tyrosine as a substrate, tyrosinase as a biocatalyst and l-ascorbate as the reducing agent. the general reaction is: tyrosinases belong to a larger group of type- copper proteins, which include catecholoxidases and oxygen-carrier haemocyanins. tyrosinases are involved in the melanin pathway and are responsible for the first steps of melanin synthesis from l-tyrosine, leading to the formation of l-dopaquinone and l-dopachrome. tyrosinases catalyse the o-hydroxylation of monophenols and the ensuing oxidation of molecular oxygen. subsequently, the o-quinones undergo non-enzymatic reactions with various nucleophiles, producing intermediates. the immobilization of tyrosinases on solid supports can increase enzyme stability, protect tyrosinase from inactivation by reaction with quinones,, improve thermal stability of fungal tyrosinases, and increase activity in comparison to soluble enzymes. diatomite is a naturally occurring, soft, chalk-like sedimentary rock that is easily crumbled into a fine, off-white powder which has k+ in the interlayer. this powder has an abrasive feeling similar to pumice and is light-weight due to its porosity. by adding diatomite into the reaction, an increased substrate uptake and enzyme production rate with concomitant l-dopa production could result. we have previously reported the effect of cresoquinone and vermiculite on the microbial transformation of l-tyrosine to l-dopa by aspergillus oryzae. in the present study, different concentrations of diatomite were added into the reaction mixture to achieve a high performance transformation of l-tyrosine to l-dopa. a. oryzae is an organism typically used for l-dopa production. the easy handling, rapid growth rate and environmentally friendly nature of alternative yeasts such as y. lipolytica have created an interest in their use for fermentation. because tyrosinases are intracellular enzymes, pre-grown cells harvested from fermented broth were used for the microbiological transformation of l-tyrosine to l-dopa.  the production of l-dopa is largely dependent on the addition of specific additives and minerals to the reaction mixture. the inductive effect of diatomite on the transformation of l-tyrosine to l-dopa by yarrowia lipolytica nrrl- was investigated. the concentration of diatomite added at the start of biochemical reaction ranged from – mg/ml, along with mg/ml l-tyrosine. a biomass concentration of mg/ml was used as a source of intracellular enzyme tyrosinase in a min reaction. the highest production of l-dopa was observed with mg/ml diatomite. l-dopa production fell while substrate consumption continued to rise, probably due to catecholase activity causing l-dopa to be used for quinone production, since ascorbic acid was not being replaced in the system. in some enzyme systems, disaccharides or higher molecular weight substrates have been found to be the best supporters of intracellular enzymes. it was hypothesized that tyrosinase, a constitutive enzyme, was altered with respect to production of l-dopa in the presence of added diatomite. the effects of delayed diatomite addition into the y. lipolytica nrrl- reaction were also investigated. reactions were performed aerobically with mg/ml cell biomass and mg/ml l-tyrosine for min. production of l-dopa increased from to min after the addition of diatomite; a significant decrease of l-dopa was noticed 20– min after the addition. maximum l-dopa was obtained min after the addition of diatomite into the reaction mixture, with concomitant tyrosine consumption of mg/ml, a 35% increase when compared to the control which is highly significant. the l-tyrosine substrate has binding affinity with diatomite, which induces tyrosinase secretion, improves its availability and ultimately leads to an increased l-dopa production rate. in our experiment, the addition of diatomite min after reaction commencement was identified as optimal, increasing production of l-dopa, substrate utilization and time of reaction. however, l-dopa production dropped when diatomite was added min after the start of reaction, probably due to conversion of unstable l-dopa to dopamine, melanin and other pigmented products after a reduced availability of the enzyme. the consumption of l-tyrosine, however, continued to increase despite the time of diatomite addition. the tyrosinase active center is comprised of dinuclear copper, coordinated with histidine residues, chelating substances or substances that are associated with this metal which are irreversible inhibitors and/or inactivators of this enzyme. the addition of diatomaceous earth may remove these inhibitors and/or inactivators by active absorption. the absorption of inhibitors increased the enzyme activity of tyrosinases, β-carboxylases and tyrosine hydroxylases which was important for the catabolism of l-tyrosine to l-dopa under controlled conditions. our data are both substantiated and in contrast to previous reports in which the production of l-dopa was achieved in minimal medium without additive supplementation. previous research efforts to produce l-dopa by the addition of μg vermiculite during the reaction obtained – mg/ml of the desired product. the time course of l-dopa production and l-tyrosine consumption was carried out at different incubation periods using a hotplate with magnetic stirrers. the control gave a maximum of mg/ml l-dopa with mg/ml consumption of l-tyrosine. the maximum conversion rate was obtained with mg/ml diatomite added min after the start of reaction, producing a 72% higher yield of l-dopa compared to the control. the l-dopa production from this time course differed significantly with the results at all other incubation periods. it is clear that up to min, cresolase activity predominated and, given the non-replacement of ascorbic acid, the overriding activity was catecholase, which consumed the l-tyrosine substrate without a corresponding production of l-dopa. after 40– min of incubation, the production of l-dopa and the consumption of l-tyrosine decreased gradually in the control and test reactions. this reduction might be because the l-dopa and residual l-tyrosine were changed into other metabolites such as dopamine, melanin and eventually melanosine. another study achieved mg/ml of l-dopa, min after the biochemical reaction. the present finding of mg/ml l-dopa after min of incubation is a major improvement. in the present study, dopamine and melanin were also produced, but their highest production was and mg/ml/h. conversion of l-tyrosine to l-dopa is an enzyme catalyzed reaction. figure shows the effect of the addition of different concentrations of drenched cell biomass on the production of l-dopa from l-tyrosine in the reaction mixture. the best results were obtained using mg/ml wet weight yeast cells, leading to fold higher productivity when compared to the control. at this concentration, most of the added tyrosine was converted to l-dopa as indicated by the small amount of residual substrate, which is highly significant. in the present investigation, the increased cell biomass enhanced enzymatic activity. however, increasing the cellular concentration beyond the optimal led to a sharp decrease in activity, probably due to increased cell concentration and the maintenance of a constant concentration of an inhibitor of catecholase activity. this product is the substrate for the second reaction catalyzed by this enzyme which leads to the formation of quinones from l-dopa. only an excessive amount of ascorbic acid continually replaced throughout the reaction might stop this second activity from taking place, leading to the formation of quinones that are also suicide inactivators of this enzyme. previous research pointed out that tyrosinase activity is directly related to the concentration of cells or mycelia in the reaction mixture in slightly acidic to neutral reaction conditions. copper atoms found at the active site of tyrosinase are an essential requirement for catalytic activity. agents such as carbon monoxide or toxins indirectly inhibit tyrosinase activity by chelating copper and abrogating its ability to bind oxygen. previous research has shown that tyrosine phenol lyase is only synthesized under l-tyrosine-induced conditions. the addition of l-tyrosine to the medium was found unavoidable when preparing cells, but severely impeded preparation of pure l-dopa. a comparison of production parameters for the effect of diatomite addition on bioconversion of l-tyrosine to l-dopa is shown in table an overall fold increase in l-dopa production was achieved at the optimal level of added diatomite when compared to the control. the optimal ph of the control reaction without added diatomite was, however, the test reaction with added diatomite was proficient at a ph range of –, indicating the enzyme remained active despite the change in reaction ph. the yp/s value was significantly improved over the control. maximum substrate consumption in terms of volumetric rate was marginally different during bioconversion between the control and test reactions, indicating maximum enzyme activity at this level of diatomite addition. the increase of qs with diatomite addition was highly significant. in the present study, the optimal values of all kinetic parameters were several-fold improved over those reported from aspergillus or cellulomonas spp.. *kinetic parameters: yp/s = mg l-dopa produced/mg substrate consumed, qs = mg substrate consumed/ml/h, qs = mg substrate consumed/mg cells/h. ** mg/ml diatomite added min after the start of reaction. $acetate buffer *$<p> is for significance level on the basis of probability. hs denotes that the values are highly significant.  in the present studies, yarrowia lipolytica strain nrrl- was exploited for l-dopa production. the addition of mg/ml diatomite markedly improved the microbiological transformation of l-tyrosine to l-dopa. diatomite addition min after the start of reaction produced a 35% higher substrate conversion rate compared to the control. a biomass concentration of mg/ml and reaction time of min were also optimized. because production of l-dopa is a high cost, low yield process, scaled up studies are a pre-requisite for commercial exploitation. 
diatomite addition min after reaction commencement improved microbiological transformation of l-tyrosine to l-dopa by y. lipolytica nrrl- a 35% higher substrate conversion rate was achieved when compared to the control. aspergillus oryzae is commonly used for l-dopa production; however, potential improvements in ease of handling, growth rate and environmental impact have led to an interest in exploiting alternative yeasts. the two important elements required for l-dopa production are intracellular tyrosinases and l-ascorbate, which acts as a reducing agent. after optimizing reaction time, and yeast cell concentration, an overall fold higher l-dopa production rate was observed when compared to the control. pre-grown cells of yarrowia lipolytica nrrl- were used for the microbiological transformation of l-tyrosine to l-dopa. the -dihydroxy phenyl l-alanine is a drug of choice for parkinson's disease, controlling changes in energy metabolism enzymes of the myocardium following neurogenic injury.
 the -dihydroxy phenyl l-alanine is a drug of choice for parkinson's disease, controlling changes in energy metabolism enzymes of the myocardium following neurogenic injury. aspergillus oryzae is commonly used for l-dopa production; however, potential improvements in ease of handling, growth rate and environmental impact have led to an interest in exploiting alternative yeasts. the two important elements required for l-dopa production are intracellular tyrosinases and l-ascorbate, which acts as a reducing agent.  pre-grown cells of yarrowia lipolytica nrrl- were used for the microbiological transformation of l-tyrosine to l-dopa. different diatomite concentrations were added to the acidic reaction mixture. maximum l-dopa biosynthesis was obtained when mg/ml diatomite was added min after the start of the reaction. after optimizing reaction time, and yeast cell concentration, an overall fold higher l-dopa production rate was observed when compared to the control. significant enhancements in yp/s, qs and qs over the control were observed.  diatomite addition min after reaction commencement improved microbiological transformation of l-tyrosine to l-dopa by y. lipolytica nrrl- a 35% higher substrate conversion rate was achieved when compared to the control. 