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Individual muscle fibers from the biceps of five indi- viduals were analyzed for 10 different enzymes. For each person, lactic dehydrogenase and adenylokinase were first measured in 50 to 100 fibers. From each large group, 20 or more fibers, representing the full range of enzyme activity, were analyzed in duplicate for the other eight enzymes (glycogen phosphorylase, P-fruc- tokinase, pyruvate kinase, glycero-P dehydrogenase, citrate synthase, fumarase, fi-hydroxyacyl-CoA dehy- drogenase, and creatine kinase). On the basis of the enzyme patterns, two clearly defined fiber types and several subtypes could be distinguished. The high lactic dehydrogenase group (type II) had high levels of other glycogenolytic enzymes and low levels of mitochondrial enzymes; the low lactic dehydrogenase group (type I) had the reverse enzyme complement. The high lactic dehydrogenase fibers consisted of two partly overlapping subgroups, containing high or me- dium adenylokinase and low or medium levels of mi- tochondrial enzymes. The low lactic dehydrogenase fi- bers, although not so clearly separable into distinct subgroups, were heterogeneous in respect to enzymes of glycogenolysis, which varied in a coordinate manner over a wide range of values. In contrast, they were comparatively homogeneous with respect to the mito- chondrial ‘enzymes. No consistent correlation was seen between creatine kinase activities and fiber type. The results demonstrate the presence among neighboring muscle fibers of a broad spectrum of highly coordinated enzyme activities and illustrate the advantages of measuring many enzymes on the same individual fi- bers. During development skeletal muscle fibers differentiate into at least three distinct types which vary in speed of contraction and enzyme composition. These may be designated slow twitch red (type I), fast twitch red (type IIA), and fast twitch white (IIB). Various other names have also been applied (l-12). (Naming the fiber types presents difficulties (8, 9).) Although most muscles contain an intimate mixture of all three fiber types, in certain muscles one or another type predominates. Enzyme analyses of these specialized muscles have shown that type I fibers are adapted to aerobic metab- olism of fats and carbohydrates, type IIB fibers are suited to anaerobic glycolysis, and type IIA fibers display both |
