Use of phospholipid-converting enzymes for the study of membrane-bound enzymes.

Conclusions Catalytic hydrogenation of the unsaturated fatty acyl residues of the lipids of sarcoplasmic reticulum to the extent of about 25% does not affect Cat+-activated ATPase activity and the process therefore is unlikely to markedly affect membrane fluidity. Since it is mainly the polyunsaturated fatty acids that are hydrogenated it may be concluded that such residues are not directly responsible for maintaining the membrane in a condition of fluidity that would affect Ca2+-dependent ATPase activities at physiological temperatures. The presence of cholesterol and rigid protein molecules in the hydrocarbon region are likely to have much greater influence on membrane fluidity in biological membranes. We may also conclude from this work that although membrane fluidity affects the activity of Ca2+-dependent ATPase of sarcoplasmic reticulum the temperatures at which discontinuities in Arrhenius plots are observed are unrelated to membrane fluidity. The possibility that temperature-dependent changes occur in the protein is clearly indicated from recent studies of the rotational motion of Ca2+-activated ATPase in native membranes (Hoffmann el al., 1979).