Abstract
A correlative histochemical study using cryostat sections and a biochemical study using isolated nuclei were undertaken to characterize nuclear adenosine triphosphate (ATP) hydrolysis in embryonic myocardium of the chick. Qualitatively, the data give an unequivocal positive correlation between the two studies except for minor sensitivity differences. A variety of criteria establish the enzymatic nature of the nuclear ATP hydrolysis and affirm the presence of at least one ATP phosphohydrolase. Enzyme activity is probably localized both at the nuclear surface and in the nucleoplasm. Mg++-activated ATP phosphohydrolase is predominant over the Ca++-activated enzyme, and the former is stimulated by the addition of NaCl. The characteristics of the enzyme activity and the combined methods employed eliminate the possibility that the observed activity is due to artifact or to contamination by other cell fractions.