Detection of fructose-3-phosphokinase activity in intact mammalian lenses by 31P NMR spectroscopy.

Recently we have identified two novel phosphorylated metabolites in the lenses of diabetic rats as sorbitol 3-phosphate (Sor-3-P) and fructose 3-phosphate (Fru-3-P). The latter compound is of particular interest since it is a potent glycating agent, which could account, at least in part, for the increased protein cross-linking and cataract formation in the lens of the diabetic rat. In order to gain insight into the mechanism of formation of these compounds, 31P NMR spectra of rat, pig, and rabbit lenses, perfused with media supplemented with glucose, fructose, or sorbitol, were acquired. Perfusion with fructose-supplemented media resulted in the production of Fru-3-P in all three species. This compound was not produced upon perfusion with fructose-deficient media. The identification of the newly synthesized material as Fru-3-P was confirmed by spiking perchloric acid extracts of the perfused lenses with synthetic Fru-3-P. Our results provide strong evidence for the existence of a fructose-3-phosphokinase in mammalian lenses. If this enzyme is present in human lenses as well, it will reinforce the hypothesis that this enzyme and its product, Fru-3-P, may play a role in diabetic cataractogenesis and that lenticular fructose-3-phosphokinase may provide another therapeutic target in the prevention and alleviation of diabetic cataracts.