Inactivation of glyceraldehyde 3-phosphate dehydrogenase by sugars, prednisolone-21-hemisuccinate, cyanate and other small molecules.

Diabetes, diarrhoea, renal failure and glucocorticoid therapy have all been identified as independent risk factors for cataract. Increased post-translational modification of proteins, leading to inactivation of enzymes and induction of conformational changes within proteins could result in lens opacification and cataract. Aspirin has been associated with many beneficial effects, including protection against cataract, in-vivo. alpha-Crystallin has been shown to act as a molecular chaperone in-vitro. This lenticular protein prevented the thermal aggregation of other lens proteins in-vitro and has sequence and functional homology with the small heat shock proteins. Glyceraldehyde 3-phosphate dehydrogenase (GAP-DH) is constitutively expressed in tissues and is susceptible to chemical modification in-vivo. In-vitro incubations of GAP-DH with sugars, cyanate and prednisolone-21-hemisuccinate, all led to significant loss of enzyme activity with time in two buffer systems. Rapid inactivation occurred when GAP-DH was incubated with fructose 6-phosphate or prednisolone-21-hemisuccinate. Slower inactivation was observed when GAP-DH was incubated with fructose, glucose 6-phosphate or potassium cyanate. Glucose did not inactivate GAP-DH under the conditions of our experiments. Aspirin and ibuprofen were shown to inactivate GAP-DH very rapidly in-vitro. Bovine lenticular alpha-crystallin conferred no protection against GAP-DH inactivation. This is the first occasion that alpha-crystallin has been demonstrated to be unable to protect against inactivation in our chemical enzyme inactivation system. This may have implications for the susceptibility of lenticular GAP-DH to post-translational inactivation.