EVIDENCE FOR EXTRACELLULAR ENZYMIC ACTIVITY OF THE ISOLATED PERFUSED RAT HEART.

1. The dissimilation of a number of externally added hexose phosphates and 5'-nucleotides by the perfused rat heart is described, and non-specific esterase and 5'-nucleotidase activity associated with the superficial cell membrane or vascular system has been demonstrated. 2. The rate of production of (14)CO(2) from [U-(14)C]glucose 6-phosphate suggests that oxidation occurred after hydrolysis to glucose. The incorporation of isotope from [U-(14)C]glucose 6-phosphate into glycogen was small, and similar to that obtained with [U-(14)C]glucose as substrate. 3. Glucose 6-phosphate was also partially isomerized to fructose 6-phosphate. Similarly, fructose 6-phosphate was converted mainly into glucose 6-phosphate, but also into glucose and inorganic phosphate. When fructose 1,6-diphosphate was added to the perfusate, a mixture of glucose 6-phosphate, fructose 6-phosphate and triose phosphates accumulated in the medium approximately in the equilibrium proportions of the phosphohexose-isomerase and triose phosphate-isomerase reactions, together with inorganic phosphate and some glucose. Glucose 1-phosphate was hydrolysed to glucose, but was not converted into glucose 6-phosphate. Leakage of enzymes out into the perfusion fluid did not occur. 4. This demonstration that phosphohexose isomerase, triose phosphate isomerase and aldolase may react with extracellular substrates at an appreciable rate suggests that these enzymes are attached to the cell membrane.