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Human cardiac ventricular myosins were prepared from autopsy samples from nine adults, seven infants, and from surgical specimens from seven patients undergoing left ventricular septal myectomy for obstructive hypertrophic cardiomyopathy. Infant myosin differed from adult myosin in two important characteristics: (a) approximately 30% of the 27,000-dalton myosin light chain is replaced by a 28,000-dalton light chain, and (b) the actin-activated myosin MgATPase activity of infant myosin is significantly lower than that of adult myosin (64 nmol phosphate released/mg myosin per min vs. 124 nmol/mg per min at 37 degrees C). The K(+)-EDTA ATPase activity of the myosin measured in 0.5M KCl is also lower in infants (1,210 nmol/mg per min vs. 620 nmol/mg per min at 37 degrees C), but the Ca(++)-activated ATPase is not significantly different. There were no differences in enzymatic activity between the normal adult and cardiomyopathic myosins.A detailed study was performed to investigate possible variations in the structure of the myosin heavy chain in infant, adult, and cardiomyopathic samples. There were no significant differences between infant and normal adult, or between normal adult and cardiomyopathic myosins seen in pyrophosphate polyacrylamide gel electrophoresis, or peptide mapping using alpha-chymotrypsin, papain, or cyanogen bromide to generate peptides. These results suggest that isoenzymes of human ventricular myosin do not exist for the myosin heavy chain in the specimens examined from infants, adults, and patients with obstructive hypertrophic cardiomyopathy. The decreased actin-activated MgATPase activity found for infant myosin appears to be due solely to a partial replacement of the 27,000-dalton light chain of myosin with a 28,000-dalton light chain.
Schier et al. (Thu,) studied this question.