Immunologically active nonapeptide fragment of a proline-rich polypeptide from ovine colostrum: Amino acid sequence and immunoregulatory properties

It has been previously found that a proline-rich polypeptide (PRP) isolated from ovine colostrum has a regulatory effect on the immune response. To study the relationship between the structure of PRP and its immunomodulatory properties, the polypeptide was digested by chymotrypsin. Products of the proteolysis were separated by gel filtration and three fractions were obtained: PRP-1, PRP-2 and PRP-3. The activity of the fractions was compared with the activity of the untreated PRP. It was found that PRP-1 was inactive, whereas PRP-2 and PRP-3 showed an activity in the regulation of the immune response assayed by measurement of PFC, and by studying effects on delayed hypersensitivity, formation of autologous rosette-forming cell, and sensitivity of thymocytes to hydrocortisone. The activity of PRP-2 and PRP-3 was comparable to the activity of PRP. The PRP-3 fraction of low mol. wt was further purified and a pure nonapeptide of mol. wt 1000 (PRP-3b) was isolated. The amino acid sequence of PRP-3b was: Val—Glu—Ser—Tyr—Val—Pro—Leu—Phe—Pro. The nonapeptide showed the full spectrum of biological activities of PRP. Comparison of terminal amino acid suggested that PRP-3b was neither the NH₂- nor the COOH-terminal fragment of PRP. The amino acid sequence of the nonapeptide indicated that PRP-3b is different from other known immunomodulators.