Activity levels of glycogen phosphorylase, hexokinase, triosephosphate dehydrogenase, lactate dehydrogenase, citrate synthase, 3-hydroxyacyl-CoA dehydrogenase, glycerolphosphate dehydrogenase (mitochondrial), and hexosediphosphatase have been determined in white (fast) muscles, red (slow) muscles, heart and smooth muscle of higher animals. The activities of these enzymes are taken as relative measures of metabolic capacities. Their ratios are interpreted as representing relations of different metabolic pathways or systems. I n all muscles investigated comparable ratios exist for phosphorylase/triosephosphate dehydrogenase (glycogenolysis/glycolysis), glycerolphosphate dehydrogenase/triosephosphate dehydrogenase (mitochondrial glycerolphosphate oxidation/glycolysis), triosephosphate dehydrogenase/lactate dehydrogenase (glycolysis/lactate fermentation), hexokinaselcitrate synthase (glucose phosphorylation/citric acid cycle) and 3-hydroxyacyl-CoA dehydrogenase/citrate synthase (fatty acid oxidation/citric acid cycle). With respect to the constancy of these ratios, consistent characteristics exist in the organization of the enzyme activity pattern. It is suggested that the more or less invariable coordination of these metabolic systems is not subject to metabolic differentiation. On the contrary, metabolic Werentiation is reflected by extreme variations of the following ratios : triosephosphate dehydrogenase/3-hydroxyacyl-CoA dehydrogenase (glycolysis/fatty acid oxidation), triosephosphate dehydrogenase/citrate synthase (glycolysis/citric acid cycle), lactate dehydrogenaselcitrate synthase (lactate fermentation/citric acid cycle), phosphorylase/hexokinase (glycogenolysis/glucose phosphorylation), and hexosediphosphatase/hexokinase (gluconeogenesis/ glucose phosphorylation). These variable enzyme activity ratios are discriminative magnitudes and make it possible to discern distinct metabolic types of muscle. White (fast) muscle is characterized by high capacities of glycogenolysis, glycolysis and lactate fermentation, whereas the capacities of glucose phosphorylation, citric acid cycle and fatty acid oxidation are low. Red (slow) muscles, heart and smooth muscle show inverse characteristics. I n white (fast) muscle, high activities of hexosediphosphatase and of mitochondrial glycerolphosphate dehydrogenase indicate that gluconeogenesis starting from glycerolphosphate or triosephosphate is probably important in this muscle type, and compensates for its low capacity of glucose phosphorylation.Enzymes. Adenylate deaminase, or AMP aminohydrolase (EC 3.5.4.6) ; adenylate kinase, or ATP: AMP phosphotransferase (EC 2.7.4.3) ; citrate synthase, or citrate oxaloacetate lyase (CoA-acetylating) (EC 4.1.3.7) ; glucosephosphate isomerase, or ~-glucose-B-phosphate ketol-isomerase (EC 5.3.1.9); glucose-6-phosphate dehydrogenase, or n-glucose-&phosphate: NADP oxidoreductase (EC 1.1.1.49); glycerolphosphate dehydrogenase, or ~-glycerol-3-phosphate: (acceptor) oxidoreductase (EC 1.1.99.5); 3-hydroxyacyl-CoA dehydrogenase, or L-3-hydro...

In biopsy samples of the lateral part of the quadriceps femoris muscle of 6 obese diabetic male patients and of 11 obese males with a normal glucose tolerance, the activities of 7 enzymes of energy metabolism were estimated: hexokinase, cytoplasmic glycerol-3-phosphate: NAD dehydrogenase, triosephosphate dehydrogenase, lactate dehydrogenase, citrate synthase, malate dehydrogenase and 3-hydroxyacyl-CoA dehydrogenase. The obese diabetic male patients exhibited decreased activities of enzymes of carbohydrate breakdown and cytoplasmic NAD regeneration. Enzymes connected functionally with aerobic metabolism were less affected. The unchanged activity of 3-hydroxyacyl-CoA dehydrogenase points to an increased role of fatty acid catabolism in the muscle.

The effect of 120-h sleep deprivation on the activity of selected enzymes of energy metabolism in skeletal muscle was studied in seven healthy volunteers. The results showed a significant decrease in the activity of malate dehydrogenase, citrate synthase, glycerol-3-phosphate dehydrogenase and lactate dehydrogenase. Triosephosphate dehydrogenase, hexokinase, and hydroxyacyl-CoA-dehydrogenase activities showed an insignificant decrease. The findings are indicative of (1) decreased aerobic oxidation capacity; (2) reduced function of reducing-equivalent carriers from cytosol across the mitochondrial membrane; (3) relative accentuation of the non-aerobic glycolytic pathway; (4) a prediabetic type of muscle metabolism.

Activity patterns of key enzymes in energy-supplying metabolism were studied in a typical fast (white) muscle (purs posterior of m. latissimus dorsi) and in a typical slow (red) muscle (pars anterior of m. latissimus dorsi) of the chicken during postnatal development. The differentiation of these enzyme activity patterns occurs postnatally, and is practically completed after 3 weeks. After hatching, the enzyme activity patterns of the two muscles resemble each other, and correspond t o that of a slow muscle, with predominantly aerobic metabolism. The slow muscle does not change its metabolic type markedly during development. The postnatal differentiation o f the fast muscle, however, implies a pronounced change of its metabolic type. It consists of a parallel increase o f absolute and specific activities of enzymes representing the following metabolic systems : glycogenolysis (glycogen phosphorylase), glycolysis (triosephosphate dehydrogenase), lactic fermentation (lactate dehydrogenase), and glycerolphosphate metabolism (glycerol-3-phosphate dehydrogenase). These concomitant changes suggest a coordinate synthesis of the respective enzymes. This conclusion applies also to a second group of enzymes which, however, decrease in absolute and specific activities. This group consists of enzymes representing the citric-acid cycle (citrate synthase), fatty-acid oxidation (3-hydroxyacyl-CoA dehydrogenase) and glucose phosphophorylation (hexokinase). Thus, enzymes belonging to constant proportion groups change in parallel during postnatal development of the muscle, and metabolic differentiation is reflected mainly by inverse shifts of constant proportion enzyme groups representing aerobic and anaerobic systems o f energy-supplying metabolism.At the level of enzymatic organization, distinct metabolic types correspond to distinct functional types of muscle [l]. From a comparison of white (fast), red (slow), cardiac and smooth muscles, it was deduced that metabolic differences a t the level of energy-supplying metabolism are reflected by certain discriminative magnitudes. These have been defined as ratios of enzyme activities representing different systems, mainly anaerobic and aerobic. Thus, the activity ratios triosephosphate dehydrogenase/3-hydroxyacyl-CoA dehydrogenase (glycolysis/fatty-acid oxidation), triosephosphate dehydrogenase/citrate synthase (glycolysis/citric-acid cycle), lactate deEnzymes. Citrate synthase, or citrate oxaloacetatelyase (CoA-acetylating) (EC 4.1.3.7) ; glycerolphosphate dehydrogenase, or ~-glycerol-3-phosphate (acceptor) oxidoreductase (EC 1.1.99.5) ; glycerol-3-phosphate dehydrogenase, or ~-glycerol-3-phosphate: NAD oxidoreductase (EC 1.1.1.8) ; 3-hydroxyacyl-CoA dehydrogenase, or L-3-hydroxyacylCoA: NAD oxidoreductase (EC 1.1.1.35); hexokinase, or ATP:D-hexose 6-phosphotransferase (EC 2.7.1.1); lactate dehydrogenase, or L-lactate : NAD oxidoreductase (EC 1.1.1.27); phosphorylase, or a-l,4--glucan: orthophosphate glucosyltransferase (EC 2.4.1 .l) ; triosephosphate dehydrogenase, or ~-glyceralde...