Printed in Great Britain Maximal activit ies o f hexokinase , 6 - phosphofructokinase , oxoglutarate dehydrogenase , and

The maximum activities of 6-phosphofructokinase and oxoglutarate dehydrogenase in muscle provide quantitative indices of the maximum capacities of anaerobic g lycolys is and the Krebs cyc le ( i .e . the aerobic capacity) respectively. These activities were measured in red, whit% and cardiac muscle of bird~ and the rat. The a c t i v i t i e s in the white pectoral muscle of the domes t i c fowl suggest t ha t the Krebs cyc le plus electron transfer could provide only about 1% of the rate of ATP production provided by anaerobic glycolysis whereas in pigeon pec to ra l muscle the p red i c t ed maximal rates from the two processes are similar. In contrast to domestic-fowl pectoral muscl% the white rat muscle, epitrochlearis, contains a significant activity of oxoglutarate dehydrogenas% which indicates that the Krebs cycle could provide about 12% of the maximum rate of ATP formation. This may be explained by a higher proportion of type-I and -IIA fibres in the rat muscle compared to the avian muscle. In the aerobic muscles of the rat the maximum activities of carnitine palmitoyl transferase indicate that fa t ty-acid oxidation could provide a high rate of ATP formation.