Identification of two arginases generated by alternative splicing in the silkworm, Bombyx mori

Arginase (EC 3.5.3.1) catalyzes the hydrolysis of arginine to ornithine and urea. Here, we have cloned two arginase cDNAs from the silkworm, Bombyx mori. The analysis of exon/intron structures showed that the two mRNAs named bmarg-r and bmarg-f were generated from a single gene by alternative usage of exons. The bmarg-r and bmarg-f were predicted to encode almost the same amino acid sequences, except that the latter had additional ten N-terminal residues. Recombinant bmARG-r and bmARG-f in Escherichia coli cell lysates were roughly similar to each other in enzymatic characteristics, which did not show large difference from those of arginases assayed by using tissue extracts. Differential RT-PCR experiments and tissue distribution analyses of arginase activity indicated that the bmarg-r gene is expressed in the male reproductive organs, especially in the glandula lacteola and vesicular seminalis, from which it is secreted to the seminal fluid and transferred to the female during copulation, whereas the bmarg-f gene is expressed in the larval and adult nonreproductive organs including the fat body and muscle, where the produced arginase proteins are considered to stay in the cells. Thus, the two silkworm arginase isoforms may have a difference in whether or not the product is excreted out of the cells in which it is synthesized.