Sulphydryl groups in photosynthetic energy conservation. IV. Inhibition of the ATPase of chloroplast coupling factor 1 by sulphydryl reagents.

1. O-Iodosobenzoate and 2,2'-dithio bis-(5-nitropyridine) inhibited by about fifty per cent the ATPase activity of heat-activated chloroplast coupling factor 1 only when present during the heating but were without effect when added before or after the activation. Reversion of this inhibition was only obtained by a second heat treatment with 10 mM dithioerythritol. 2. The inhibition of the Ca2+-ATPase of coupling factor 1 by o-iodosobenzoate or 2,2'-dithio bis-(5-nitropyridine) was not additive with similar inhibitions obtained with the alkylating reagents iodoacetamide and N-ethylmaleimide. 3. The heat-activated ATPase of o-iodosobenzoate-treated coupling factor 1 had a higher Km for ATP, without modification of V. The modified enzyme was desensitized against the allosteric inhibitor ADP.