Age and the control of glycolysis in the rat lens

Previous studies with lens dispersions indicated that the rate-limiting step in glycolysis shifts from hexokinase (HK) in the young lens to phosphofructokinase (PFK) in older lenses. Because the concentrations of the complex controlling factor for these enzymes could not be reproduced reliably in homogenates, the question of age-related control of glycolysis was re-examined in intact lenses. Toward this end, the levels of several metabolites of glucose were measured in fresh and incubated clear lenses. Of the substrates measured per fresh lens, only one changed significantly with age; fructose diphosphate was increased. When lenses were incubated in 2 to 12 mM glucose, the lactate production per lens was not significantly different with age. Together these results suggested that the glycolytic mass of the lens was constant with age. In both young and older lenses, increases in glucose in the medium led to increases in both glucose and glucose-6-phosphate in the lens. The lack of corresponding increase in lactate production suggested that the regulatory step lay downstream from HK, probably at PFK. This finding was corroborated by evidence that the initial acceleration of lactate production by the addition of cyanide (the Pasteur effect) was accompanied by decreases in the substrates of PFK, glucose-6-phosphate and fructose-6-phosphate. A secondary disinhibition of HK, as indicated by decreased lens glucose, became apparent after longer incubation with cyanide. This suggested that after disinhibition of PFK, HK became rate-limiting until the level of glucose-6-phosphate fell enough to allow the disinhibition of the latter enzyme as well. Thus PFK seemed to be the primary regulatory step in aerobic glycolysis in lenses of rats from 1 to 12 months of age.