The ultrastructural changes of S-100 protein localization during lipolysis in adipocytes. An immunoelectron-microscopic study.

To elucidate the changes of ultrastructural localization of S-100 protein during lipolysis in adipocytes, an immunoelectron-microscopic study was performed. Epididymal fat pads from Wistar rats were incubated in the buffer with or without 10 microM epinephrine. Before incubation with epinephrine, S-100 protein was found to be associated with closely packed polysomes, the membrane of microvesicles, plasma membranes, the outer membrane of mitochondria, and the pellicle around fat droplets. In the epinephrine-treated tissues, however, S-100 protein-positive polysomes decreased drastically. S-100 protein-positive microvesicles increased in number, lined up below the plasma membranes, and fused with the plasma membrane, frequently opening into the interstitium. These microvesicles were also found around the lipid droplets. These findings, together with those of a previous report on ultrastructural changes of adipocytes during lipolysis, suggest that S-100 protein molecules interact with free fatty acids (FFAs) on their hydrophobic portions on the membrane of microvesicle and then are translocated through the cytoplasm and discharged from the surface of plasma membranes with FFAs into the interstitium. That is, S-100 protein might serve as one of carrier proteins of FFAs in adipocytes.