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ANUMBER of enzymes are known to occur in the body in one or more distinct forms, usually called isozymes.1-3 The molecular basis for this phenomenon is known in the case of two soluble enzymes: creatine kinase and lactic dehydrogenase. In both cases the isozymes are formed of polymers of two kinds of subunits, each of which is under separate genetic control.Both enzymes, more particularly lactic dehydrogenase, have come under intensive investigation in the past few years, in part because of the inherent interest of the phenomenon of isozyme structure, and also because it has been hoped that the pattern of isozymes in a cell would give information or insight of clinical value.This paper reports the isozyme types and normal levels of creatine kinase from human tissues obtained at autopsy. These values are of interest because of the widespread use of serum creatine kinase activity as an indicator

INTRODUCTION From the standpoint of the relative activity of a number of enzymes in individual muscle fibers, the histochemical studies presented in a companion article1 disclosed the presence of eight types of fibers in the calf muscles of the rat. The various types of fibers formed a continuum which could be arbitrarily divided into three groups. The gastrocnemius and plantaris were found to be composed of fibers of all the eight types. Within the gastrocnemius, the different types of fibers were found to have a definite topographical arrangement, some portions containing a majority of fibers of group I and other portions a predominance of fibers of group II. The soleus consisted entirely of fibers of group III.One of the aims of the presently reported combined histochemical and quantitative study was to obtain quantitative data on the activity of various enzymes in the three groups of fiber types. For

Clinical and experimental observations have established that damaged or degenerating muscles will release its contents, particularly enzymes, into the general circulation, and that this process can be measured by an increase in the level of certain enzymes in the serum. Assay of serum enzymes in the assessment of muscle disease has been employed most widely in the study of muscular dystrophy both to establish the diagnosis early1,2 and to discover maternal carriers of the sex-linked form.3,4 However, it is clear that at least some muscle enzymes are released into serum by other myopathies, particularly acute myositis,5,6 and even by neurogenic atrophy.5,6That the source of the serum enzymes is indeed the muscle has been established by a number of findings. Dreyfus et al7 showed that the venous aldolase level was sometimes slightly higher than the arterial aldolase in cases of muscular dystrophy. The isoenzymes of

INTRODUCTION THE DIFFERENCES in mammalian skeletal muscle metabolism with higher lactic acid production in white muscle13,17,18 and increased oxygen consumption2,10,37 and carbon dioxide evolution3 in red muscle indicate a greater reliance of the former upon anaerobic reactions for the production of energy, and of the latter upon aerobic mechanisms. The demonstration by Lawrie29,30 of the direct relationship between the activity of the cytochrome system and the content of myoglobin in striated muscle focused attention upon the variation in enzyme activity with pigmentation. Bücher and his colleagues7,39 studying several enzymes in the skeletal muscles of rabbits and rats have shown distributions of enzyme activities favoring glycolysis in white muscles and oxidative processes in red. Recent studies from this laboratory8,42 with histochemical and quantitative methods showed such enzymatic differences not only between different muscles, but between various types of muscle fibers within a muscle.