1. The hexokinase activity of homogenates of eggs and embryos of the sea urchin Arbacia punctulata has been measured. Expressed as micrograms glucose consumed at 20°C., per hour per milligram of protein the following values were obtained: unfertilized eggs, 67; fertilized eggs, 72; 24 hour plutei, 94; 48 hour plutei, 226. The concentration of the enzyme in the eggs is small and may be calculated to be about 0.001 per cent of the dry weight of unfertilized eggs.

2. The hexokinase activity of the egg homogenate was virtually all recovered in the supernatant fraction when the homogenate was centrifuged at 20,000 x g for 30 minutes and was found to have the following properties: The concentrations for half maximal hexokinase activity with various substrates were, approximately: Glucose, 0,00003 M; fructose, 0.00075; mannose, 0.00007; 2-desoxyglucose, 0.00025. The relative rates of phosphorylation of various sugars by the supernate fraction when saturated with substrate were, approximately: Glucose, 1.0; mannose, 1.2; fructose, 1.8; 2-desoxyglucose, 2.0; glucosamine, 0.6. Adenosinediphosphate and glucose-6-phosphate inhibited the enzyme. No evidence for more than one hexokinase in the Arbacia extracts was found.

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