MAXIMUM RATE OF OXYGEN-UPTAKE BY HUMAN SKELETAL-MUSCLE IN RELATION TOMAXIMAL ACTIVITIES OF ENZYMES IN THE KREBS CYCLE

1. Ten subjects performed incremental exercise up to their maximum wor k rate with the knee extensors of one leg. Measurements of leg blood f low and femoral arteriovenous differences of oxygen were made in order to be able to calculate oxygen uptake of the leg. 2. The volume of th e quadriceps muscle was determined from twenty-one to twenty-five comp uter tomography section images taken from the patella to the anterior inferior iliac spine of each subject. 3. The maximal activities of thr ee enzymes in the Krebs cycle, citrate synthase, oxoglutarate dehydrog enase and succinate dehydrogenase, were measured in biopsy samples tak en from the vastus lateralis muscle. 4. The average rate of oxygen upt ake over the quadriceps muscle at maximal work, 353 ml min(-1) kg(-1), corresponded to a Krebs cycle rate of 4.6 mu mol min(-1) g(-1). This was similar to the maximal activity of oxoglutarate dehydrogenase (5.1 mu mol min(-1) g(-1)), whereas the activities of succinate dehydrogen ase and citrate synthase averaged 7.2 and 48.0 mu mol min(-1) g(-1), r espectively. 5. It is suggested that of these enzymes, only the maximu m activity of oxoglutarate dehydrogenase can provide a quantitative me asure of the capacity of oxidative metabolism, and it appears that the enzyme is fully activated during one-legged knee extension exercise a t the maximal work rate.