The findings during this past year strongly support our hypothesis that pyrroline-5-carboxylate (P5C) is a novel mediator of intercellular communication and transmembrane signaling. First, we showed that P5C stimulates the turnover of membrane phosphoinositides. With the P5C stimulation of PRPP production in intact cells as an endpoint, we found a pattern of synergism with specific growth factors and sensitivity to inhibitors consistent with the interpretation that P5C produced its effect by activating protein kinase C at a point proximal to the production of diacylglycerol. Related and perhaps linked to this mechanism for transmembrane signaling, a novel uptake mechanism for P5C was identified and characterized. This mechanism is specific, carrier mediated, energy dependent, but sodium independent. Importantly, P5C is converted to proline concomitant to cellular entry. This group translocation directly couples the uptake of P5C to the transfer of redox within or at the plasma membrane. Finally, the functional association of P5C translocation to P5C reductase, the enzyme that mediates redox transfers, has been corroborated with the direct demonstration of the enzyme's association with plasma membranes. Using sucrose density gradients, we showed that P5C reductase, previously considered to be cytosolic, is, in fact, associated with cellular organelles. One component is associated with plasma membranes and a second with mitochondria. The membrane component has 7-fold higher affinity for NADPH than the mitochondrial component.
