Retinal hexokinase: kinetic properties and the effect of cyclic 3',5'-adenosine monophosphate.
We measured hexokinase (EC 2.7.1.1) activity in particulate and soluble fractions isolated from bullfrog (Rana catesbeiana) retinas. Seventy-three percent of the hexokinase (HK) activity was associated with the particulate fraction, 27% with the soluble fraction. Both HK fractions could phosphorylate fructose, glucose, 2-deoxy-D-glucose, and mannose, but not galactose. The Km for glucose was 0.14 mM, for 2-deoxy-D-glucose, 3.6 mM. With glucose as substrate, the Vmax for particulate HK was 125-148 microM retina-1 min-1, for soluble HK, 37 microM retina-1 min-1. Product inhibition of particulate HK activity by glucose 6-phosphate was marked, whereas 2-deoxy-D-glucose 6-phosphate did not inhibit the activity. Cyclic AMP stimulated the HK activity of both retinal fractions nearly twofold at concentrations of 0.2-0.8 mM; AMP was much less effective in this regard.
D000818:Animals; D000242:Cyclic AMP / Q000494:pharmacology*; D019298:Glucose-6-Phosphate; D005958:Glucosephosphates / Q000494:pharmacology; D006593:Hexokinase / Q000378:metabolism*; D006601:Hexoses / Q000378:metabolism; D007700:Kinetics; D011892:Rana catesbeiana; D012160:Retina / Q000201:enzymology*; D012995:Solubility; D013379:Substrate Specificity; D014018:Tissue Distribution
D005958:Glucosephosphates; D006601:Hexoses; D019298:Glucose-6-Phosphate; D000242:Cyclic AMP; D006593:Hexokinase