VITAMINS

Cellular retinol-binding protein, type II (CRBP(II)) is present in large quantities in small intestine, and may play an important role in absorption and metabolism of retinol and β-carotene. The involvement of CRBP (II) in retinol absorption and its esterification in enterocytes have been proven in our several studies. The CRBP (II) levels in the small intestine were very low in embryonic chicks and then increased rapidly around hatching. Retinol bound to CRBP (II) is esterified by lecithin : retinol acyltransferase (LRAT) which activity was also extremely low in the embryos, but rose rapidly around hatching in parallel with the induction of intestinal CRBP (II). Also CRBP (II) was evidenced its role in preventing the toxic effect of unbound retinol n the rat small intestine, suggesting that consumption of excess vitamin A in amounts below 10 times the NRC'c requirement may not cause toxic disturbance. W found that the jejunum-bypass operation led to increases in the amounts of both CRBP (II) and lipoprotein Apo-B in the residual jejunal segment, suggesting that CRBP (II) might be enhanced by high fat absorption. Indeed, we discovered that rat CRBP (II) mRNA was enhanced by high fat (corn oil) diet. This increase was due to unsaturated fatty acids, but its mRNA levels were not directly related to the expression of RXRα and peroxisome proliferator-activated receptor (PPARα). We concluded that dietary fatty acids modify the CRBP (II) gene expression.

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Pituitary adenylate cyclase-activating polypeptide (PACAP) is a polypeptide hormone which stimulates adenylate cyclase in pituitary cells. Two forms of PACAP are known, one of 38 amino acids (PACAP-38) and a C-terminally truncated form of 27 amino acids (PACAP-27). PACAP-27 has an amino acid sequence identity of 68% with vasoactive intestinal polypeptide (VIP) and of 37% with secretin, indicating that PACAP is a member of the VIP/secretin/glucagon family. PACAP is present not only in various areas of the central nervous system including the hypothalamus, cerebral cortex, hippocampus and posterior pituitary, but also in peripheral tissues such as the testis, adrenal gland and gut, and seems to function as a neuromodulator or neurotransmitter in the central and peripheral nervous systems. PACAP exerts its actions through G-protein linked membrane receptors. Recently, three receptor subtypes have been cloned : PACAP receptor is a PACAP-preferring receptor, while VIP receptor and VIP2 receptor exhibit similar affinities for PACAP and VIP. Together with receptors for secretin, glucagon, glucagon-like peptide 1, growth hormone-releasing factor, calcitonin, and parathyroid hormone/parathyroid hormone-related peptide they constitute a subfamily of the Gs protein-coupled receptors. The PACAP-R gene has recently been isolated. The gene contained alternative exons which are processed by different splicing and give isolated. the gene contained alternative exons which are processed by different splicing and give rise to several receptor variants. Expression of these variants revealed altered properties of ligand binding and cAMP and phosphatidylinositol production. The distribution of the PACAP and VIP receptor mRNAs was examined in rat tissues by Northern hybridization and in situ hybridization. The distribution of PACAP receptor mRNA differed considerably from, though partly overlapped, that of VIP receptor mRNA, suggesting that these receptors have different functions. Possible roles that these receptors might play were discussed.

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