Biosynthesis of rat liver peroxisomal serine:pyruvate aminotransferase.

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In rat liver, there are two types of serine:pyruvate aminotransferase (SPT) as to the organelle distribution. One is a mitochondrial enzyme (SPTm) and the other a peroxisomal enzyme (SPTP). The properties of the two enzymes are apparently indistinguishable, but administration of glucagon or insulin causes the selective induction of SPTm. We have shown previously that SPTm is biosynthesized from 1900 nt-mRNA as 45 kDa-precursor which is then specifically translocated into mitochondria and converted therein to mature SPTm. In this study, the precesses involved in the biosynthesis of SPTp were investigated. Results obtained are as follows: 1. On RNA blot analysis, 1700 nt-mRNA was detected in addition to 1900 nt-mRMA. The structure of the two mRNAs were extremely similar except that 1700 nt-mRNA lacks about 70 nucleotides of 5'-terminal sequence of 1900 nt-mRNA and has about 100 nucl eotides shorter poly-A tail when compared with 1900 nt-mRNA formed shortly after the administration of the hormones.2. Southern blot analysis of rat genomic DNA showed that the SPT gene is single. The analyses on primer extension and S1 nuclease mapping using DNA fragment of the genomic clone revealed that 1900 nt- and 1700 nt-mRNAs are transcribed from different initiation sites in the same exon 1. 3. In cell-free translation, 43 kDa-product was detected in addition to 45 kDa-product. the 43 KDa-product was suggested to be related to SPTp bv a similar response to hormones or peroxisome proliferators. However, the 43 kDa-product was bound to but not taken up into peroxisomes in vitro under conditions so far tested. 4. The N-terminal sequence of SPTm was determined to be Met-Gly-Ser-His---. The N-terminal Met corresponded to the initiation Met in the translation of 1700 nt-mRNA, suggesting that SPTm and SPTp share the same primary structure, although their biosynthetic processes and hence the organelle Colalization differ.