Effect of Protein Depletion and Repletion in the Rabbit on the Activity of S35 from Methionine in Serum Proteins and Urine

Sulfur-35 from injected methionine was measured in serum protein and urine of adult rabbits after dietary protein depletion and after repletion. Five rabbits fed a 4% casein diet for 39 days developed a hypochromic microcytic anemia, lost weight and were in negative nitrogen balance; however serum protein concentra tions and fractions and blood volumes showed no significant change. During the same period there were no significant differences in these parameters between 2 groups of rabbits fed either a 20% casein ration or standard chow. In the depleted animals serum protein S35activity 8 hours after injection was higher, specific activity of the albumin fraction increased over 18 days, the percentage of the total activity persisting in the a-globulin fraction was greater and more S35/mg of urinary nitrogen was excreted although the total urinary S35 was less. When the depleted animals were fed the 20% casein ration for 34 days, differences in nitrogen balance, red cell indexes, body weight and the utilization of the isotope disappeared. These results indicate that the isotope detected alterations in serum proteins that were not apparent by changes in concentration or albumin and globulin fractions. Before protein deficiency results in functional impairment and symptoms be come clinically manifest, adaptive mech anisms are utilized by the body in an effort to maintain homeostasis ( 1). These mechanisms serve to preserve the nor malcy of traditional indicators of protein deficiency until severe deficiency is pres ent. Blood volume changes may mask alterations in serum protein concentra tions, serum fractions and red cell in dexes. Loss of body weight may be obscured by edema. Positive nitrogen retentions occur in the presence of depleted nitrogen stores (2). The diffi culty in evaluating marginal deficiency is documented by the inconsistency of re sults in studies attempting to evaluate protein nutriture. Even when the subject is used as his own control for observations on protein deficiency, changes may not be detected by the more commonly employed tools of evaluation (3). For the past 10 years radioactive tracers have been used for compartmental analy sis and for study of the rates of synthesis and degradation of serum proteins (4, 5). Garrow (6, 7) was one of the first to em ploy a tracer amino acid, S35-L-methionine, for the purposes of clinical evaluation of protein nutriture. Allison et al. (8) reported an increase in the specific radioactivity of serum pro teins in tumor-bearing and protein depleted dogs. Others (9-11) have also reported the preferential incorporation of S35-Lmethionine into certain tissues in states of deprivation. Alteration in the patterns of incorpora tion of S35from methionine by the serum proteins may be apparent before the usual signs of protein deficiency occur. If so, protein deficiency would be detectable be fore the appearance of the more profound changes necessary at present to classify a person or animal as protein-deficient. The purpose of the present study was to develop mild-to-moderate protein de ficiency in rabbits and to observe the effects of this on the rate of uptake of Received for publication September 11, 1964. 1The material presented is taken in part from the dissertation submitted by Gladys W. Strain in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Western Reserve University. 2This work was supported in part by Public Health Service Research Grant no. HD-00669. 3This work was presented in part at the annual meeting of the Federation of American Societies for Experimental Biology at Chicago, Illinois, 1964. 4Formerly Predoctoral Fellow, National Institutes of Health, Public Health Service. J. NUTRITION, 85: '65 407 by gest on O cber 4, 2011 jn.nition.org D ow nladed fom 408 GLADYS W. STRAIN, HAROLD B. HOUSER AND HELEN A. HUNSCHER S35 from methionine by the serum pro teins, the incorporation of the tagged amino acid into the serum protein frac tions, and the urinary excretion pattern of the radioactive label.