The isoform composition and types of functioning of Na+,K(+)-ATPase complexes, as well as their ouabain-inhibition constants, were studied for calf brain membranes. The catalytic subunit alpha 3 within the native enzyme complex was found to exhibit an increased sensitivity to endogenous proteolysis. The site of specific proteolysis was localized in the region of the polypeptide chain that is unique for all alpha 3 type isoforms: PNDNR492 decreases (Y493) (according to the numeration of human alpha 3-subunit). It was shown for the first time that in all enzyme preparations containing the alpha 2 and alpha 3 isoforms isolated by both Jorgensen's and Esmann's method two other proteins were present: the beta 5 chain of tubulin and glyceraldehyde-3-phosphate dehydrogenase; the biological meaning of their association is still unclear.