BINDING OF GLUCOSE, GALACTOSE AND PYRIDOXAL-PHOSPHATE TO LENS CRYSTALLINS

Glycation of proteins plays an important role in diabetic complications. Both glucose and galactose were shown to bind progressively to lens crystallins with decreased binding in the presence of increasing concentrations of pyridoxal 5-phosphate (PLP). In longer term incubations (10 mM sugar for 21 days) glucose produced no significant yellowing of the protein, that is no detectable advanced glycation products, but pyridoxal phosphate (15 mM) caused an increased absorbance at 325 nm. This increase was greater in the presence of glucose. It appears that PLP becomes firmly attached to the protein and that this binding is enhanced in the presence of glucose or galactose. Changes produced by sodium borohydride indicate that the PLP is attached to protein amino groups as a Schiff base. Incubation of lens crystallins with PLP also led to increased fluorescence which was greater when sugar was present. However, borohydride experiments indicated that glucose and galactose may decrease the formation of non-reducible adducts of PLP. The decreased glycation in the presence of PLP supports the notion that it might be useful in prevention of diabetic complications, but the reaction of PLP itself with protein is less encouraging.