Do the soluble glutathione S-transferases have direct access to membrane-bound substrates?

The ability of the soluble glutathione S-transferases to bind the membrane (liposome) bound substrates 1-chloro-2,4-dinitrobenzene and sulfobromophthalein was determined. The transferases were found to have access only to substrates in the aqueous phase. They could not not bind membrane-bound substrates and, thus, enzymatic activities were reduced by the membrane partitioning of the substrates. The reduction in enzymatic activity was directly proportional to the lipid solubility of the substrate. The liposomes had no direct effect on the enzyme per se. [35S]Sulfobromophthalein and [14C]chlorodinitrobenzene bound to liposomes were found to have rapid rates of release into the aqueous phase. Rates of hydration of chlorodinitrobenzene from liposomes were rapid enough such that rates of catalysis (measured in a stopped-flow spectrophotometer) were affected only by the partition coefficient of substrate between lipid phase and water, and not by the rate of transfer of substrate from lipid to water phase.