From: asc7556@u.cc.utah.edu (Andrew Corradini/SBDC)
Subject: Re: Left-handed sugar
Keywords: Chemists Please. . . ~~~~~~~~~~~~~~~~~~~~
Organization: University of Utah Computer Center Student Mail Machine
Date: Tue, 5 Jan 93 05:48:39 GMT

In article <1992Dec27.125851.17259@lclark.edu> degraw@lclark.edu (David Degraw) writes:
>
>In article <72463@cup.portal.com>, mmm@cup.portal.com (Mark Robert Thorson)
>writes:
>
>>>Please excuse the cross-posting into the sci newsgroups, but we've been
>>>having a discussion in rec.food.cooking about the supposed existence
>>>of left-handed sugars which are indigestible hence calorie-free.
>>>I contend that if L-sugars were calorie-free, they probably wouldn't
>>>be sweet, either.  Can someone in the sci groups clarify this?
>
>By L-sugar, I suppose you mean the levorotatory variety of an enantiomer.
>Common table sugar is sucrose, an enatiomeric molecule.  The stuff
>we eat is a racemic (50%-50%) mixure of R and L sucrose.  It may be 
>possible to isolate L-sucrose by using a *certain* microorganism which 
>can metabolize only R-sucrose.  A technique I read about to isolate
>alpha-L-glucose used a microorganism (I don't have the refernce for which
>kind) which selectively metabolized alpha-R-glucose.
>
>I cannot tell you which enantiomer of sucrose actually reacts with human
>taste receptors.
>
>Both R and L sucrose should be digestible since only a fraction of human
>digestion is carried out by bacteria.  Most of the digestion of 
>sucrose occurs in the small intestine where it is lysed by sucrase into
>glucose and fructose.  Glucose and fructose are then transported to the 
>liver for storage via the intestinal capillaries.  Although most of the
>bacteria live in the small intestine, they seem to play a small role in
>sucrose digestion

This sounds real scientific and all, although you use several spellings of
enantiomer/enatiomer and I think the use of "levorotatory" was probably
just a typo ;-) of levorotary...  Terminology notwithstanding...
 
Glucose and fructose each occur in both dextro- and levo-rotary isomers. 
The resulting sucrose is merely a compound of the two.  By "occur", I
should say, *could* occur, since in nature, all simple sugars (e.g.,
glucose, fructose, galactose, etc.) are found exclusively in the dextro-
form.  The formation of DNA, and thus the cellular mechanisms responsible
for sugar formation, are exclusively dextro-rotary.  Elsewise, we would
have organisms capable of digesting only those of equivalent "polarity",
or (pardon the liberty), "sugar-sex".
 
If the two types of sugars occurred in nature equally (pardon me:
"racemically"), only half the sugar in any box/ice-cream-cone/cake would
be digestible, since the greatest part of sugar metabolism is carried out
intra-cellularly, and human enzymes (as with all Earth critters) are
exclusively dextro-rotary.  The referenced posting was full of really nice
information and neat-o bioterminology; the conclusion, however, was
completely wrong-o.

I regret having no citations at hand, but I did extensive research as an
undergrad on this topic, since it seemed like a good
get-rich-reasonably-quick scheme if you could find a differentiatory
"sieve" for L-sugars.  Many commercial labs have *produced* (*NOT*
extracted!) L-sugars, but the cost is as of yet prohibitive.)
 
Sorry, David,

Andrew


Newsgroups: rec.food.cooking