Regulatory Mechanisms in Carbohydrate Metabolism

Inhibition of mammalian hexokinase by glucose g-phosphate was first reported by Weil-Malherbe and Bone (1) and studied by Crane and Sols (2). An inhibition of rabbit muscle phosphofructokinase by excess adenosine triphosphate was observed by Lardy and Parks (3) and was shown to be reversed by a number of intracellular components by Passonneau and Lowry (4). Similar findings were reported on the enzyme from liver fluke (5), heart muscle (6), and yeast (7). A possible role for glucose-6-P inhibition of hexokinase in the Pasteur effect was proposed many years ago (8), but it was realized that by itself it did not represent a sufficient control mechanism. When the inhibition of P-fructokinase by ATP was show-n to be reversed by inorganic orthophosphate (4), the possibility of a sequential control mechanism was considered and a reinvestigation of the Pasteur effect in reconstructed systems was init,iated. While this investigation was in progress, a stimulation of hexokinase by Pi (9) was shown to be due to a reversal of the glucose-6-P inhibition (10). Previous experiments (11) on reconstructed systems were performed with crystalline yeast hexokinase. Since this enzyme does not exhibit a product inhibition by glucose-6-P, it was necessary to undertake the purification and study of a suitable mammalian enzyme. It is the purpose of this paper to describe the purification and some properties of hexokinase and P-fructokinase from mammalian t.issues. The succeeding paper (12) describes studies with these enzymes in reconstructed systems.