Somatomedin production by rat liver in organ culture. II. Studies of cartilage sulphation inhibitors released by the liver and their separation from somatomedins.

Inhibitors of cartilage sulphation have been found to be released by the rat liver in organ culture. They cause a decrease in [35S]sulphate uptake by embryonic chick cartilage and, when added to a constant amount of serum, counteract the somatomedin (SM) activity of the serum. Both of these effects are dose-dependent. Their antagonistic action, investigated in the presence of increasing concentrations of serum, appeared to resemble non-competitive inhibition which would suggest different sites of action for SM and inhibitors. Incubation of the liver explants with cortisol (0.01-1 microgram/ml) increased the sulphation-inhibiting activity of the culture medium and the effect was dose-dependent. Simultaneous addition of cycloheximide suppressed the inhibition. Gel filtration of the culture medium on Sephadex G 75 showed that: a) at pH 7.9, inhibitors eluted in the same fractions as [125I]SM-A bound to its carrier (apparent molecular weight approximately 45 000); b) at pH 2.4, inhibitors still eluted as large molecules, but SM activity appeared in the same fractions as the dissociated [125I]SM-A. The question arises whether the cartilage sulphation inhibitor might not be the same molecule as the SM-carrier protein released by the liver.