Turnover of the catalytic subunit of Na,K-ATPase in HTC cells.

A polyclonal antibody to the catalytic subunit of rat kidney Na,K-ATPase has been raised in rabbits and used to analyze the turnover of the subunit in the rat hepatoma cell line HTC. It had been shown previously (Baumann, H., and Doyle, D. (1978) J. Biol. Chem. 253, 4408-4418) that the membrane proteins of these cells displayed multicomponent turnover kinetics, the minority of the surface proteins turning over with a half-time of about 20 h and the remainder with a half-time of about 100 h. That the antibody precipitated both the alpha (catalytic) and beta (glycosylated) subunits of the Na,K-ATPase from Triton extracts of HTC cells could be demonstrated following metabolic labeling of the cells with either [3H]leucine or a mixture of [3H] mannose and [3H]fucose, but following labeling with [35S]methionine radioactivity was found only in the alpha subunit of the precipitates. Incorporation of [35S]methionine into the alpha subunit could be detected 2 min after addition of the isotope to the cell suspension. Then and at all times thereafter the label was recoverable only from the particulate fraction of a 150,000 X g 60-min centrifugation; no labeled alpha subunit was ever detected in the supernatant fraction. By quantitative densitometry of radioautographs of sodium dodecyl sulfate-polyacrylamide gels of labeled antibody precipitates, it could be shown in pulse-chase experiments that the specific activity of the alpha subunit remained unchanged for 3-4 h (transit time) after the pulse was initiated and that the activity subsequently decayed exponentially with a half-time of 18 h. In a population growing with a generation time (tG) of 33 h, this decay corresponds to a turnover rate constant of 0.49/tG. The catalytic subunit is among those membrane proteins with a rapid turnover rate.