The long-term goals of our research are directed towards an understanding of the mechanism by which enzymatic activities are modified by the binding of soluble proteins to insoluble membranes (allotopy). The partially resolved transhydrogenase system of R. rubrum is being recognized as consituting one portion of the active site for transhydrogenation while the membrane binding site is the complementary region. The factor can carry out a partial reaction of transhydrogenation (the NADH reduction of 3AcPyAD ion) but requires the membrane for the catalysis of complete transhydrogenation (NADPH reduction of NAD ion). Interestingly, the partial reaction is inhibited by the antifolate metabolite methotrexate. The significance of this finding is under current investigation. The catalytic transhydrogenation reaction is made up of the combination of the soluble protein factor and the membrane. Since the factor undergoes association-dissociation during catalysis, the active site is of a temporal but dynamic existence. This dynamic aspect of the active site is believed to be a unique phenomenon with respect to catalysis but may be of general occurrence with respect to catalysis at membrane surfaces. The characteristics of the active site are to be studied by a combination of physical and chemical approaches. The transhydrogenase factor is composed of two subunits and it is considered that one unit may be responsible for the specificity of binding to the membrane and the other for the catalytic transformation. These aspects are to be investigated by chemical modification and immunological studies. Further resolution of the membrane is directed towards the isolation of the transhydrogenase binding region of the membrane.