Folia Biologica
Journal of Cellular and Molecular Biology, Charles University 

Apolipoprotein J (apoJ) is a 70 kDa glycoprotein associated with high-density lipoproteins (HDL) in human plasma (de Silva et al., 1990a). HDL transport more than 80% of the plasma cholesterol from extrahepatic tissues to the liver for excretion (Glomset, 1968). Wide tissue distribution of apoJ was determined by isolation of apoJ mRNA from a variety of human tissues. The presence of apoJ mRNA was mainly detected in the steroidogenic tissues, testis and ovary, and also in the brain (de Silva et al., 1990b). The apoJ molecule is a dimer which consists of two disulfide-linked subunits designated apoJα (34-36 kDa) and apoJβ (36-39 kDa) (de Silva et al., 1990c). For purification and characterization of the apoJ molecule, monoclonal antibodies were utilized. Six monoclonal antibodies specific for apoJ were generated and described (de Silva et al., 1990a, 1990c). ApoJ has extensive similarities to the human protein designated as SP-40,40 (or complement lysis inhibitor (CLI)) (Choi et al., 1989) and human TRPM-2/clusterin (Wong et al., 1994). The presence of SP-40,40 was also demonstrated within human seminal plasma at levels comparable to those in serum (Kirszbaum et al., 1989). Both these proteins, SP-40,40 and TRFM-2/clusterin, have been implicated in a variety of physiological processeses, including sperm maturation, lipid transport, membrane remodelling and inhibition of the complement cascade (Wong et al., 1994). ApoJ is also secondarily incorporated into the sperm membrane, as sperm travel through the male reproductive tract, and belongs to the sperm-coating proteins. Among monoclonal antibodies against human sperm proteins that we generated, a monoclonal antibody that specifically recognized apoJ was found. The monoclonal antibody was designated Hs-3.

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