Variable distribution of three molecular forms of peptide histidine isoleucinamide in rat tissues: identification of the large molecular form as peptide histidine valine-(1-42).

We previously isolated three forms of peptide histidine isoleucinamide (PHI) from rat small intestine by chromatography on Fractogel. Rat PHI-(1-27)-NH2 and rat PHI-(1-27)-Gly were identified by microsequence analysis. In the present work the third larger form was purified to homogeneity, being detected by RIA in preliminary steps and by RRA in the last steps. This form could be recognized by a PHI antiserum and an antiserum raised against the synthetic rat PHI-(22-27) fragment C-terminally extended by the connecting peptide linking PHI to vasoactive intestinal peptide (VIP) in their common PHI/VIP precursor, as postulated by the reported cDNA analysis of rat brain mRNA. Sequence analysis and C-terminal hydrolysis by carboxypeptidase-Y identified this large form as peptide histidine valine-(1-42), i.e. rat PHI-(1-27) C-terminally extended by the totality of the connecting peptide except for the terminal dibasic residues. We also documented the coexistence of the three PHI forms in rat brain, rachidian bulb, pituitary, adrenal glands, uterine horns, and stomach. Tissue distribution was highly variable. The classical PHI-(1-27)-NH2 form was best represented in rachidian bulb and somewhat less so in brain and uterine horns. PHI-(1-27)-Gly, while being particularly abundant in the small intestine, was very poorly present in rachidian bulb and uterine horns. Peptide histidine valine-(1-42) was the major form in pituitary and adrenal glands and was also well expressed in uterine horns. The three forms coexisted equally in stomach. This uneven distribution suggests a tissue-specific posttranslational processing of rat prepro-PHI/VIP.