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Cytochemical localization of transferase activities: carnitine acetyltransferase.

Two methods for the cytochemical detection of free CoA and their utilization in the fine-structural localization of carnitine acetyltransferase in rat heart are described. The first utilizes the reducing property of the SH group of CoA to reduce potassium ferricyanide to potassium ferrocyanide, which in the presence of uranyl ions forms an electron-dense precipitate of uranyl ferrocyanide. The second utilizes the mercaptide-forming property of the free SH group of CoA, which forms a precipitate with cadmium ions. Using the uranyl-ferrocyanide method, reaction product due to endogenous enzymic activity was found on and between the cristae and between the inner and outer membranes of the mitochondria in fresh heart muscle. In aldehyde-fixed tissue activity was recorded only between the inner and outer membranes. Endogenous activity was removed by preincubation of the tissue in a solution of ferricyanide. On addition of acetyl CoA and carnitine to the incubation medium, fresh tissue, which had been preincubated in ferricyanide, showed reaction product between and on the cristae and between the inner and outer membranes of the mitochondria, while fixed tissue showed reaction product in the latter position only. In both cases the activity between the outer and inner mitochondrial membranes was dependent on both acetyl CoA and carnitine, while the cristae reaction occurred in the absence of carnitine, but required acetyl CoA. All activity was inhibited by mercuric chloride. Acetyl carnitine reduced the activity in the fixed tissue and had severe effects on the structure of fresh mitochondria. These results suggest the presence of carnitine acetyltransferase, which survives aldehyde fixation, on the inner surface of the outer mitochondrial membrane and/or the outer surface of the inner mitochondrial membrane. A second enzyme which released CoA from acetyl CoA occurred in relation to the cristae of unfixed mitochondria. The cadmium method was less satisfactory than the uranyl-ferrocyanide method but with fixed tissue gave confirmatory results.