Abstract
Phosphoglucoisomerase (EC 5.3.1.9. PGI), phosphoglucomutase (EC 2.7.5.1. PGM) and glucose-6-phosphate-dehydrogenase (EC 1.1.1.49. G6PDH) compete for the substrate G6P. Besides others it depends on these enzymes in which direction G6P is metabolized (glycolysis, glycogen-cycle, pentose-pathway). G6P concentration was measured in intact bovine lenses as well as in single lens parts in dependence on age. Moreover, the specific activities of the 3 enzymes and their Km-values were determined. PGM and G6PDH show almost identical Km-values (1.5 to 3.5 · 10-5M) in intact lenses as well as in all parts investigated, with the exception that G6PDH is not present in the lens nucleus. The special activity with 1 to 3 mU per total lens is also similar in both enzymes. PGI shows a Km of 1.5 · 10-4M with a specific activity of 300 mU per total lens. Neither PGI nor PGM seems to be rate-limiting even in the aging lens, whereas the decreasing G6PDH activity effects the pentose pathway and thus the NADP/NADPH ratio mainly in the lens nucleus.
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