Effectors of rat-heart hexokinases and the control of rates of glucose phosphorylation in the perfused rat heart

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Abstract
1. The kinetic properties of the soluble and particulate hexokinases from rat heart have been investigated. 2. For both forms of the enzyme, the Km for glucose was 45μm and the Km for ATP 0·5mm. Glucose 6-phosphate was a non-competitive inhibitor with respect to glucose (Ki 0·16mm for the soluble and 0·33mm for the particulate enzyme) and a mixed inhibitor with respect to ATP (Ki 80μm for the soluble and 40μm for the particulate enzyme). ADP and AMP were competitive inhibitors with respect to ATP (Ki for ADP was 0·68mm for the soluble and 0·60mm for the particulate enzyme; Ki for AMP was 0·37mm for the soluble and 0·16mm for the particulate enzyme). Pi reversed glucose 6-phosphate inhibition with both forms at 10mm but not at 2mm, with glucose 6-phosphate concentrations of 0·3mm or less for the soluble and 1mm or less for the particulate enzyme. 3. The total activity of hexokinase in normal hearts and in hearts from alloxan-diabetic rats was 21·5μmoles of glucose phosphorylated/min./g. dry wt. of ventricle at 25°. The temperature coefficient Q10 between 22° and 38·5° was 1·93; the ratio of the soluble to the particulate enzyme was 3:7. 4. The kinetic data have been used to predict rates of glucose phosphorylation in the perfused heart at saturating concentrations of glucose from measured concentrations of ATP, glucose 6-phosphate, ADP and AMP. These have been compared with the rates of glucose phosphorylation measured with precision in a small-volume recirculation perfusion apparatus, which is described. The correlation between predicted and measured rates was highly significant and their ratio was 1·07. 5. These findings are consistent with the control of glucose phosphorylation in the perfused heart by glucose 6-phosphate concentration, subject to certain assumptions that are discussed in detail.