Role of free L-carnitine and acetyl-L-carnitine in post-gonadal maturation of mammalian spermatozoa

Abstract

Spermatozoa are produced in the testis and undergo post-gonadal modifications in the epididymis to acquire fertilizing ability. In epididymal plasma, high-molecular-weight proteins and such small molecules as free-L carnitine convert the gametes into "competent' and functional cells. This review summarizes the knowledge pertaining to L-carnitine and the significance of free L-carnitine uptake into the mature spermatozoa of mammals. We provide an overview of the function of free L-carnitine and carnitine esters in the metabolism of eukaryotic cells and review the role of the specific carnitine acyltransferases in mitochondrial transport of fatty acids and in modulating acyl-coenzyme A (CoA) pools in cellular organelles. In mammals, including man, free L-carnitine is taken from blood plasma and concentrated in the epididymal lumen. This epididymal secretion is beneficial for spermatozoa and is not merely an excretory waste. The uptake of free L-carnitine into the spermatozoa and its metabolic outcome are discussed first in in-vivo and then in in-vitro situations. Free L-carnitine goes through the sperm plasma membrane by passive diffusion. Free L-carnitine is acetylated in mature spermatozoa only. The excess acetyl-CoA from the mitochondria is probably stored as acetyl-L-carnitine and modulates the reserves of free CoA essential to the function of the tricarboxylic acid cycle. These properties of L-carnitine of buffering CoA in the mitochondrial matrix are known in somatic cells but are accentuated in this study of the male germinal cells. In the future, a precise measurement of the in-vivo and in-vitro concentrations of free CoA and acetyl-CoA in the cellular compartments of immature and mature spermatozoa might complete these data. The relationship between the endogenous pools of free and acetylated L-carnitine and the percentage of progressive sperm motility indicates a more important metabolic function related to flagellar movement. In conclusion, the potential to initiate sperm motility, which takes place in the epididymis, is probably independent of the carnitine system, while the energy properties of acetyl-L-carnitine can only be relevant in situations of "energy crisis'. The uptake of "cytoplasmic' free L-carnitine in mature spermatozoa must be a protective form of mitochondrial metabolism, useful to the survival of this isolated cell.