Studies on Myo-Inositol Metabolism in Galactosemia

Abstract

During the investigation of galactitol accumulation in the tissues of galactosemia patients, it was observed that the concentration of free and lipid-bound myo-inositol in the brain was lower than that in normal brain.l.2 At various stages of development, brains from rats fed a diet containing 35% galactose contained somewhat lower concentrations of inositol than did those from comparable control^.^ These findings suggested a possible relationship between galactosemia and myo-inositol metabolism. Because of the possible toxicity of the accumulation of galactitol in brain tissues from galactosemic rats or humans, Dr. Henry J. Wells began a study of the effect of galactitol on the particulate system from rat brain which incorporates inositol into the lipid fraction, as described by Thompson et al.4 The incubating medium consisted of microsomes prepared from brains of young male adult rats of the Holtzman strain; CTP (1.0 mM); MnCl, (2.0 mM); phosphatidic acid (0.125 pmoles) dispersed in Tween-80 (7.5 p g ) ; 2-14C-myoinositol (3.3 x lo8 cpm) ; tris, pH 7.4 (50 mM); potassium phosphate, pH 7.4 (10mM); and galactitol, either omitted or in variable amounts of 0.032-0.25 pmole in a final volume of 0.3 ml.The incubations were carried out at 37" C for two hours.Under the conditions of this assay, it appeared that added galactitol inhibited inositol incorporation into the lipid fraction. However, based on radioactivity measurements, the incorporation of myo-inositol into phosphatidylinositol for control samples was only approximately 0.03%. Further study of this reaction has been carried out, and the results from a number of experiments revealed only mild inhibition ( 10-20% ) when inhibitor concentrations were five times that of substrate; higher concentrations of galactitol did not increase the inhibition. These results suggested that the poor yields of phosphatidylinositol led to the discrepancies observed. We then turned to the method of assay for phosphatidylinositol described by Prottey and Hawthorne5 for the mammalian pancreas enzyme. This assay differed from that of Thompson et al. by utilizing larger amounts of crude microsomes and did not require added phosphatidic acid because of the high endogenous quantities of phosphatidic acid in the microsomal fraction. Albumin was also added in the latter assay procedure although no substantial differences were found when it was omitted. Linear results were obtained when activity (mpmoles phosphatidylinositol produced) was plotted against microsomal protein concentration (0.1-0.8 mg) . Using substrate concentrations in the first order region of the two-step reaction, it was found that galactitol was not inhibitory over a wide range of concentrations. Galactose-1-phosphate and galactose-6-phosphate were likewise not in-*

Myoinositol and phosphatidylinositol metabolism in synaptosomes from galactose-fed rats.

“…The possibility exists, therefore, that the low concentrations of these compounds in whole brain may be a reflection of a defect in glia or in the proliferation of glial cells during the period of active myelination. In this respect, it is interesting to note that when rats are first introduced to a high galactose diet on weaning at 19 days, the marked fall in Ins concentrations in brain cannot be reproduced (14). Thus, it is possible that once the period of rapid glial proliferation has begun, the brain may be more resistant to the toxic effects of galactose or its metabolites at least with respect to Ins metabolism.…”

Oxidative stress contributes to outcome severity in a Drosophila melanogaster model of classic galactosemia

“…These include ATP depletion via futile cycles of phosphorylation and dephosphorylation of galactose (Mayes and Miller, 1973), inhibition of key enzymes by galactose-1-phosphate (gal-1P) (Wells et al, 1969; Gitzelmann, 1995; Parthasarathy et al, 1997; Bhat, 2003) and depleted UDP-gal leading to impaired galactosylation of cerebrosides (Lebea and Pretorius, 2005). …”

Galactosemia: The good, the bad, and the unknown

“…Alternatively, Gitzelmann (1995) suggested that elevated intracellular gal-1P might inhibit a number of important enzymes, including glucose-6-phosphatase, phosphoglucomutase, liver glycogen phosphorylase, UDP-glucose pyrophosphorylase, glucose 6 phosphate dehydrogenase, and at high concentrations, UDP-gal galactosyltransferase. Although the data remain controversial, other reports suggest that elevated gal-1P may also inhibit myoinositol monophosphatase (Wells et al, 1969;Parthasarathy et al, 1997;Bhat, 2003), the enzyme principally responsible for the production of myo-inositol in many tissues. If true, this inhibition could explain the dramatic loss of both free and lipid-bound myo-inositol observed in brain samples from galactosemia patients (Wells et al, 1965).…”