Serum CPK in muscular dystrophy and myotonia dystrophica

Creatine phosphokinase (CPK) activity in serum was measured in control subjects and in patients with various forms of muscular dystrophy or with myotonia dystrophica by the one enzyme backward ward reaction (C + ATP ⇌ CP + ADP) without and with the addition of a sulfhydryl compound (mercaptoethanol). In healthy subjects and in patients without myopathy or neuropathy the addition of the sulfhydryl compound to such assays based on one enzyme did not raise the mean serum CPK value (differences recorded without or with mercaptoethanol were not statistically significant), nor did it alter the range of minimum-maximum values. In terms of reference standards serum CPK values lie between zero and 16.9 I.U. in 99 per cent of control subjects and the remainder is between 17 and 40.9 I.U. The addition of mercaptoethanol to the one enzyme backward reaction, judging from our studies, increases the mean value of CPK in patients with pseudohypertrophic and limb girdle muscular dystrophy and decreases the overlap of these groups with control values. Similar trends in FSH dystrophy and myotonia dystrophica were not statistically significant, but our observations are limited in number. Comparisons of our data based on the one enzyme backward reaction with results from laboratories employing this plus two other enzyme reactions indicate that both methods can yield serum CPK values of the same order of magnitude. Despite differences in the magnitude of serum CPK activity reported by various laboratories employing a variety of assay methods, there is general agreement concerning the frequency and the course of increased serum CPK activity in the various primary myopathies, myotonia dystrophica, and in neurologic disorders.