Inactivation of Pneumococcal Hemolysin by Certain Sterols.

I. It is shown that the inhibiting effect on pneumolysin by cholesterol and certain related sterols is apparently determined primarily by the presence of the OH group in the sterol structure and secondarily by the double bond. Possible peroxide foimation would seem to account for only a small portion of the cholesterol effect. 2. Active pneumolysin is inhibited promptly, while inactive (air-oxidized) lysin is affected slowly if at all by cholesterol. Likewise, active lysin is adsorbed by red cells, while the iiiactive form is not. That is, the state of oxidation of the lysin conditions its reactivity with the sterol and with its adsorption on red cells. 3. The free thiol grouping on the active lysin molecule remains free after treatment with excess cholesterol. The lytic activity, therefore, is associated with at least 2 functional groupings, one reversibly oxidizable, and the other more or less specific for a certain sterol grouping and configuration.