The Metabolism of Acetylcarnitine and Acetate by Bovine and Hamster Epididymal Spermatozoa1

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Abstract
Since acetylcarnitine has been identified in the epididymal plasma of many mammalian species, we investigated whether acetylcarnitine could serve as an energy substrate for epididymal bull and hamster spermatozoa. Intact caudal cells from both species oxidized [1-14C]acetyl-l-carnitine to 14CO2, in vitro, and the amount oxidized was dependent on time, substrate concentration, and cell number. Within each species, the rate of oxidation was the same as the rate at which free [1-14C]acetate was oxidized. Spermatozoa incubated with [3H]acetyl-L-carnitine hydrolyzed the compound and [3H]acetate accumulated in the medium. Unlabeled acetate added to the incubation medium, competed with cellular uptake of [3H]acetate and resulted in further increase in [3H]acetate accumulation in the medium. Furthermore, the acetyl group of acetylcarnitine was oxidized by spermatozoa without concomitant uptake of the carnitine group. Purified plasma membrane vesicles contained an acetylcarnitine hydrolase activity that was solubilized from whole cells by detergents and that could be distinguished from acetylcholinesterase also present in the cells. The solubilized acetylcarnitine hydrolase activity was inhibited by p-hydroxymercuriphenylsulfonate, but not by the specific acetylcholinesterase inhibitors, eserine or BW63C47. The sulfhydryl blocker also inhibited the production of 14CO2 from [1-14C]acetylcarnitine by intact cells; acetylcholinesterase inhibitors did not. From estimates of sperm energy requirements, our results indicate that extracellular acetylcarnitine serves as a physiologically important energy substrate for maturing sperm cells.