Human lens enzyme alterations with age and cataract: glyceraldehyde-3-P dehydrogenase and triose phosphate isomerase.

The isoelectric point distribution of G-3-P DH and TPI from human lenses was examined as a function of age and cataract formation. Both enzymes exhibited progressive heterogeneity with age and a shift towards an acidic charge. Little qualitative differences in the pI profiles of G-3-P DH and TPI were found to distinguish mixed cataracts from age comparable normal lenses. While the most alkaline form of G-3-P DH required less HAsO4= for optimal activity, no other kinetic property, i.e. Km substrate, cofactor and inhibitors distinguished any of the charge forms of G-3-P DH. All metaor isozyme forms of TPI had the same Km substrate in the forward and reverse reaction direction. The most acidic forms of G-3-P DH and TPI were less stable to increased temperatures than their more alkaline counterparts suggesting a decreased stability.

Purification and properties of human cataractous lens glyceraldehyde-3-phosphate dehydrogenase.

Glyceraldehyde-3-phosphate dehydrogenase has been shown to occur in three different forms in the human adult cataractous lens: a membrane bound form (M) and at least two cytosolic isozymes: I1 and I2. Similar Km's for substrate, cofactor and HAsO4 were established for each form and all three forms, to differing degree, require a reduced sulfhydryl group for maximum activity. A variety of phosphonucleosides (ATP, ADP, AMP and 3' 5' cyclic AMP) as well as NADH inhibit enzyme activity. Inhibition by ATP is non-competitive whereas cyclic AMP and NADH compete for the cofactor binding site. Chloride ion stimulates and inhibits enzyme activity at low and high concentrations respectively.