I would be interested to find out information on this substance (which I
assume is a polysaccharide) - in particular I would like to know the best
natural plant sources and I would also be interested to know what
concentrations (very approximately) it occurs in fruit and vegetables.

Many Thanks.

The name that you are giving is not a modern identifier of a specific
chemical (at least not to my various references). I can only assume that
this is one of the 9 sterioisomers of inositol, of which several occur
naturally.
Although inositol is known as "meat sugar", it is not a polysaccharide.
It is a cyclohexane (6 carbon ring) with every carbon having both an OH
and an H attached to it.
Inositol occurs widely in both animals and plants (part of the fiber as
phytic acid or inositol hexaphosphate), but I have no idea which if any
sources contain more of any particular stereoisomer.

--Tom
Tom Matthews

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  I think that most, if not all, of the inositol in nature is
1,2,3,5/4,6 inositol, also known as i-inositol, meso-inositol or
myo-inositol.  As the meso indicates, this molecule has no
stereoisomers, because the mirror image can be converted to the
original by simply swapping it around, head to tail.  Thus, it is its
own mirror image.

    OH       OH
    |-------|  OH
   / OH      \|        1,2,3,5/4,6 hexahydroxyhexane (myo-inositol)
  |\|_______ /
  OH        |
            OH  

   In the body, derivitives of myo-inositol can be phosphated to second
messagers or reacted with lipids in many ways, and these compounds
often are stereoactive (generally D) and the body's enzymes can tell
the stereoisomers.  But the sterospecificity in in these compunds only,
not in the inositol that goes to make them.  Cleave off the
substituents and you get plain old m-inositol again.

   Incidentally, lindane (gammexane) is 1,2,3,5/4,6
(gamma)hexachlorohexane, the same meso molecule, but with chlorines
instead of hydroxyls.  It apparently interfers with insect m-inositol
metabolism.

   There is no such single substance.  The common inositol in your body
and in plants is myo-inositol and isn't chiral.  There are D-chiral
inositol compounds in humans and plants, but the chirality is not due
to the inositol itself.  These are still substituted m-inositols, and
only become chiral because of the asymetry of the substituents with
regard to the 2--6 symmetry axis.  Take these off and you still have
m-inositol.

    Where did you find the term, and why do you ask?

Perhaps the following abstract from the April 29, 1999 issue of NEJM
could shed some light on this mysterious substance:

John E. Nestler, Daniela J. Jakubowicz, Paula Reamer, Ronald D. Gunn,
Geoffrey Allan. Ovulatory and Metabolic Effects of d-Chiro-Inositol in the
Polycystic Ovary Syndrome. The New England Journal of Medicine -- April
29, 1999 -- Vol. 340, 1314-20.
http://www.***.com/

    "Abstract

    Background. Women with the polycystic ovary syndrome have
    insulin resistance and hyperinsulinemia, possibly because of a
    deficiency of a d-chiro-inositol-containing phosphoglycan that'
    mediates the action of insulin. We hypothesized that the
    administration of d-chiro-inositol would replenish stores of the
    mediator and improve insulin sensitivity.

    Methods. We measured steroids in serum and performed {*filter*}
    glucose-tolerance tests before and after the {*filter*}administration of
    1200 mg of d-chiro-inositol or placebo once daily for six to eight
    weeks in 44 obese women with the polycystic ovary syndrome. The serum
    progesterone concentration was measured weekly to monitor for
    ovulation.

    Results. In the 22 women given d-chiro-inositol, the mean (SD) area
    under the plasma insulin curve after the {*filter*}administration of
    glucose decreased from 13,41711,572 to 51586714 U per milliliter
    per minute (8169 to 3140 nmol per liter per minute) (P=0.007;
    P=0.07 for the comparison of this change with the change in the
    placebo group); glucose tolerance did not change significantly. The
    serum free testosterone concentration in these 22 women decreased from
    1.10.8 to 0.50.5 ng per deciliter (3828 to 1717 pmol per
    liter) (P=0.006 for the comparison with the change in the placebo
    group). The women's diastolic and systolic {*filter*} pressure decreased
    by 4 mm Hg (P<0.001 and P=0.05, respectively, for the comparisons with
    the changes in the placebo group), and their plasma triglyceride
    concentrations decreased from 18488 to 11061 mg per deciliter
    (2.10.2 to 1.20.1 mmol per liter) (P=0.002 for the comparison with
    the change in the placebo group). None of these variables changed
    appreciably in the placebo group. Nine{*filter*} of the 22 women who
    received d-chiro-inositol ovulated, as compared with 6 of the 22 women
    in the placebo group (P<0.001).

    Conclusions. d-Chiro-inositol increases the action of insulin in
    patients with the polycystic ovary syndrome, thereby improving
    ovulatory function and decreasing serum androgen concentrations, {*filter*}
    pressure, and plasma triglyceride concentrations. (N Engl J Med
    1999;340:1314-20.)"

This study is reported in the article

New Drug May Help Women With Infertility Syndrome
http://www.***.com/ :80/dg/fa2f2.htm

Additionally, I foun the following web link:
D-CHIRO-INOSITOL AND INSULIN ACTION
Richard E. Ostlund Jr., M.D.
http://www.***.com/ ~rescolla/res_collab97/html/D-CHIRO-INOSITO...

A citation:

  "Almost all of the inositol in nature is myo-inositol. However,
   several preparations of phosphoglycan molecules that appear
   to mediate insulin action as second messengers contain
   D-chiro-inositol, an epimer of myo-inositol that was unknown in
   mammals until recently. This laboratory has shown that diabetic
   subjects excrete 5-40 times as much D-chiro-inositol in the urine as
   normals and that the rate of excretion depends critically on the degree
   of diabetic control. Plasma D-chiro-inositol levels are influenced by
   insulin treatment and are a strong predictor of hyper-triglyceridemia,
   the most common lipid abnormality in diabetic patients.
   D-chiro-inositol can be obtained from the diet in the form of pinitol,
   a methyl inositol found in legumes. Feeding diabetic patients
   D-chiro-inositol from soy results in reductions in plasma insulin
   levels. The effects of D-chiro-inositol on diabetic subjects is being
   studied.

   The exact structure of the chiro-inositol-containing molecules thought
   to be responsible for these effects has not been reported. Negative ion
   gas chromatography/mass spectrometry and insulin bioassays can be used
   to prepare candidate biochemical fractions. Electrospray ionization
   (ESI) and matrix-assisted laser desorption ionization (MALDI) mass
   spectrometry are being employed to determine a structure for the intact
   mediator molecules."

--
Matti Narkia

I got my above information, from the Merck Index so I expect it is
correct.
Therefore, I expect the 'if not all' is incorrect.
But, yes, I goofed. I should have said (as Merck did) "there are nine
*possible* steroisomers, of which seven are optically inactive or meso."

Merck states: "Two optically active forms, the racemic form, and several
cis,trans-isomers occur naturally", with the prevalent natural form
being the cis-1,2,3,5-trans-4,6-cyclohexanehexol which they draw
essentially as below (except for using the 'wedge' 3-d representation
which unfortunately cannot be done in ASCII).

--Tom
Tom Matthews

The LIFE EXTENSION FOUNDATION - http://www.lef.org - 800-544-4440
A non-profit membership organization dedicated to the extension
of the healthy human lifespan through ground breaking research,
innovative ideas and practical methods.
LIFE EXTENSION MAGAZINE - The ultimate source for new
health and medical findings from around the world.

meso."

     OH       OH
     |-------|  
    / OH      \        
   |\|_______ /|
   OH        | OH
             OH

   Comment:  Yes, it seems I goofed as well.  I had thought there were
two pairs of stereoisomers, but can find only one.  The D,L pair
consists of the one above, and its mirror image.  I don't know which of
this chiral pair is D, but it seems that "D-inositol" (the "chiro" is
redundant) is indeed enough to specify which of the 9 isomers of
inositol one is talking about.  The seven remaining meso isomers have

1:  All OH's "up"
2:  One up, 5 down
3:  Two up (1,2) four down (3,4,5,6)
4:  Two up (1,3) four down (2,4,5,6)
5:  Two up (1,4) four down (2,3,5,6)
6:  Three up (1,2,3)  three down (4,5,6)
7:  Three up (1,3,5)  three down (2,4,6)