Isolation, characterization, and corticotropic activity of rabbit adrenocorticotropin

Rabbit (r) ACTH was extracted from 600 pituitaries, and 2 forms of immunoreactive ACTH were identified with the least polar form accounting for approximately 90% of the total. Peptide mapping and sequence analysis indicated that three tryptic peptides had retention times identical to those obtained from human (h) ACTH. The least polar tryptic fragment from rACTH had a shorter retention time than the corresponding one from hACTH. Sequence analysis indicated that rACTH differed from hACTH at three different loci, namely, an Asn in place of Asp in position 29; a Val in place of Leu in position 37, and a Val in place of Phe in position 39. Biological activity of the ACTH was compared with synthetic hACTH in 2 bioassays with adrenals from 10-day-old pups, the first using dispersed rabbit adrenal cells and the second using monolayer adrenal cells in culture. The biological potencies of the two ACTH preparations were identical with respect to corticosterone (B) release in the short term bioassay, with an ED50 value of 1.67 X 10(-10) M. The ED50 value for cortisol (F) release for rACTH and hACTH were 1.1 X 10(-10) M and 1.67 X 10(-10) M, respectively, which were not statistically different. The biological potency of rACTH in the monolayer adrenal cell system for both F and B was significantly greater than the hACTH, and the ED50 values were 4.4 X 10(-10) M and 8.9 X 10(-10) M, respectively. There was a progressive decrease in the B/F ratios with increasing concentrations of ACTH in both the bioassay systems suggesting that ACTH stimulated the 17 alpha-hydroxylase activity even when the exposure of cells to ACTH was as short as 2 h.