Spermatozoan maturation, motility, and fertility are, in part, dependent upon the progressive increase in epididymal and spermatozoal carnitine, critical for mitochondrial fatty acid oxidation, as sperm pass from the caput to the cauda of the epididymis. We demonstrate that the organic cation/carnitine transporters, OCTN1, OCTN2, and OCTN3, are expressed in sperm as three distinct proteins with an expected molecular mass of 63 kDa, using Western blot analysis and our transporter-specific antibodies. Carnitine uptake studies in normal control human sperm samples further support the presence of high-affinity (OCTN2) carnitine uptake (K-m of 3.39 +/- 1.16 muM; V-max of 0.23 +/- 0.14 pmol/min/mg sperm protein; and mean +/- SD; n = 12), intermediate-affinity (OCTN3) carnitine uptake (K-m of 25.9 +/- 14.7 muM; V-max of 1.49 +/- 1.03 pmol/min/mg protein; n = 26), and low-affinity (OCTN1) carnitine uptake (K-m of 412.6 +/- 191 muM; V-max of 32.7 +/- 20.5 pmol/min/mg protein; n = 18). Identification of individuals with defective sperm carnitine transport may provide potentially treatable, etiologies of male infertility, responsive to L-carnitine supplementation. (C) 2003 Elsevier Science (USA). All rights reserved.