A 17.5 kDa protein was isolated from porcine whey by reverse phase HPLC and identified as a putative whey acidic protein (WAP) homologue by sequencing 35 and 40 amino acid residues of the amino- and carboxy-terminus respectively. Degenerate oligonucleotides to both of these amino acid sequences were designed and used in reverse transcriptase PCR with RNA from lactating porcine mammary gland as a template. A 162 bp PCR fragment was detected and sequenced. Compilation of the deduced and determined amino acid sequence revealed a protein of 111 amino acids, which had approximately 75, 50, 40 and 35% similarity at amino acid level to camel, rabbit, rat and mouse WAP respectively. It also included the four-disulphide core characteristic of all WAP proteins and most Kunitz-type protease inhibitors. This provides the first unequivocal evidence for WAP secretion in the pig. SDS PAGE analysis of the whey fraction showed that WAP is secreted as a major protein in sow's milk from farrowing to weaning. The molecular mass of WAP in SDS PAGE was significantly greater than the 11.7 kDa determined from amino acid sequence, indicating that porcine WAP is possibly glycosylated. Northern analysis detected a single mRNA transcript of approximately 600 bp in porcine RNA from the mammary gland of lactating sows. To examine the hormone-regulated expression of the WAP gene the mammary glands of sows at day 90 of pregnancy were biopsied and explants cultured for 3 days in the presence of various combinations of porcine insulin (I), cortisol (F) and porcine prolactin (P). Northern analysis of RNA extracted from the tissue indicated that WAP gene expression was barely detectable in the mammary gland prior to culture and there was no increment in explants cultured in the presence of I and F. However, a significant increase in the accumulation of WAP mRNA was observed in explants cultured in I, F and P. A similar result was observed for beta-casein and alpha-lactalbumin gene expression.

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