Taibi Gennaro, Di Gaudio Francesca, Nicotra Concetta M A
Dipartimento di Oncologia Sperimentale e Applicazioni Cliniche, Universita di Palermo, Palermo, Italy.
J Enzyme Inhib Med Chem. 2008 Jun;23(3):317-27. doi: 10.1080/14756360701584539.
Retinoic acid is considered to be the active metabolite of retinol, able to control differentiation and proliferation of epithelia. Retinoic acid biosynthesis has been widely described with the implication of multiple enzymatic activities. However, our understanding of the cell biological function and regulation of this process is limited. In a recent study we evidenced that milk xanthine oxidase (E.C. 1.17.3.2.) is capable to oxidize all-trans-retinol bound to CRBP (holo-CRBP) to all-trans-retinaldehyde and then to all-trans-retinoic acid. To get further knowledge regarding this process we have evaluated the biosynthetic pathway of retinoic acid in a human mammary epithelial cell line (HMEC) in which xanthine dehydrogenase (E.C. 1.17.1.4.), the native form of xanthine oxidase, is expressed. Here we report the demonstration of a novel retinol oxidation pathway that in the HMEC cytoplasm directly conduces to retinoic acid. After isolation and immunoassay of the cytosolic protein showing retinol oxidizing activity we identified it with the well-known enzyme xanthine dehydrogenase. The NAD+ dependent retinol oxidation catalyzed by xanthine dehydrogenase is strictly dependent on cellular retinol binding proteins and is inhibited by oxypurinol. In this work, a new insight into the biological role of xanthine dehydrogenase is given.
视黄酸被认为是视黄醇的活性代谢产物，能够控制上皮细胞的分化和增殖。视黄酸的生物合成已被广泛描述，涉及多种酶活性。然而，我们对这一过程的细胞生物学功能和调控的理解是有限的。在最近的一项研究中，我们证明了乳黄嘌呤氧化酶（E.C. 1.17.3.2.）能够将与细胞视黄醇结合蛋白结合的全反式视黄醇（全反式视黄醇 - 细胞视黄醇结合蛋白复合物）氧化为全反式视黄醛，然后再氧化为全反式视黄酸。为了进一步了解这一过程，我们评估了人乳腺上皮细胞系（HMEC）中视黄酸的生物合成途径，其中黄嘌呤脱氢酶（E.C. 1.17.1.4.），即黄嘌呤氧化酶的天然形式，是表达的。在此我们报告了一种新的视黄醇氧化途径的证明，该途径在HMEC细胞质中直接导致视黄酸的生成。在分离并免疫测定显示视黄醇氧化活性的胞质蛋白后，我们将其鉴定为著名的酶黄嘌呤脱氢酶。黄嘌呤脱氢酶催化的NAD + 依赖性视黄醇氧化严格依赖于细胞视黄醇结合蛋白，并被氧嘌呤醇抑制。在这项工作中，我们对视黄嘌呤脱氢酶的生物学作用有了新的认识。
