Tissue-Specific Distribution of a Peroxisomal 46-kDa Protein Related to the 58-kDa Protein (Sterol Carrier Protein X; Sterol Carrier Protein 2/3-Oxoacyl-CoA Thiolase)

Abstract The complete sequence of the nonspecific lipid-transfer protein (nsL-TP; sterol carrier protein 2) including the presequence is present at the C-terminus (residues 405–547) of a 58-kDa protein. To be able to study this 58-kDa protein without the interference of nsL-TP, antibodies were raised against predicted epitope regions in the N-terminal part (peptide I, residues 23–43; peptide II, residues 130–149). Using these antibodies, rat tissues were analyzed by immunoblotting. In rat liver, in addition to the 58-kDa protein the antibody against peptide I (α-58K 23 ) as well as the antibody against peptide II (α-58K 130 ) detected a 46-kDa protein. This suggests that both peptide sequences are present in this 46-kDa protein. Both the 46- and the 58-kDa-proteins were abundantly present in liver and adrenals, but could also be detected in brain, kidney, heart, lung, testes, and ovary. This distribution was observed in tissues from both male and female rats. Immunogold labeling of cryosections of liver showed that α-58K 23 labels the peroxisomes. From double-labeling experiments using α-nsL-TP and α-58K 23 we conclude that the 46-kDa protein is peroxisomal. We propose that in the peroxisomes the protease that processes pre-nsL-TP also cleaves the 58-kDa protein giving rise to the 46-kDa protein and nsL-TP. In addition to the 58- and 46-kDa proteins, an immunoreactive 44-kDa protein was prominently present in rat heart and at low levels also in small intestine and brain. Immunogold labeling of cryosections of heart and Western blotting of purified mitochondria showed that the 44-kDa protein is localized in the mitochondria. The 44-kDa protein was shown to be identical to mitochondrial sarcomeric creatine kinase, which has a peptide segment of five amino acid residues in common with peptide I.