The in vitro metabolism of 13-cis-retinoic acid in a model sebaceous structure, the rat preputial gland

Role of the molybdoflavoenzyme aldehyde oxidase homolog 2 in the biosynthesis of retinoic acid: generation and characterization of a knockout mouse

The mouse aldehyde oxidase AOH2 (aldehyde oxidase homolog 2) is a molybdoflavoenzyme. Harderian glands are the richest source of AOH2, although the protein is detectable also in sebaceous glands, epidermis, and other keratinized epithelia. The levels of AOH2 in the Harderian gland and skin are controlled by genetic background, being maximal in CD1 and C57BL/6 and minimal in DBA/2, CBA, and 129/Sv strains. Testosterone is a negative regulator of AOH2 in Harderian glands. Purified AOH2 oxidizes retinaldehyde into retinoic acid, while it is devoid of pyridoxal-oxidizing activity. Aoh2(-/-) mice, the first aldehyde oxidase knockout animals ever generated, are viable and fertile. The data obtained for this knockout model indicate a significant role of AOH2 in the local synthesis and biodisposition of endogenous retinoids in the Harderian gland and skin. The Harderian gland's transcriptome of knockout mice demonstrates overall downregulation of direct retinoid-dependent genes as well as perturbations in pathways controlling lipid homeostasis and cellular secretion, particularly in sexually immature animals. The skin of knockout mice is characterized by thickening of the epidermis in basal conditions and after UV light exposure. This has correlates in the corresponding transcriptome, which shows enrichment and overall upregulation of genes involved in hypertrophic responses.

Chromatography of retinoids

This article reviews the determination of retinoic acids and their metabolites (first-generation retinoids), aromatic retinoids (second generation) and arotinoids (third generation) in biological samples. Because of the sensitivity of the retinoids to isomerization and oxidation, special care has to be taken from sample collection and storage, throughout extraction, till the final chromatographic separation. High and strong protein binding, and insolubility in aqueous solutions hamper the extraction from biological samples. Various extraction procedures are discussed, mainly involving liquid-liquid extraction of biological fluids or lyophilized tissue samples. The new technique involving direct injection of biological fluids or tissue homogenates, using high-performance liquid chromatography (HPLC) with automated column switching, provides full protection from light and simplifies sample work-up. HPLC with ultraviolet detection is the method of choice for the determination of retinoids, because it is rapid, sensitive and allows separation of geometric isomers and metabolites within a wide polarity range. Gas chromatography-mass spectrometry is not appropriate for first- and second-generation retinoids because of isomerization, but allows very sensitive determination of third-generation retinoids, although very extensive sample clean-up and derivatization are necessary. However, direct injection of large volumes of biological fluids into HPLC systems, using on-line solid-phase extraction and automated column-switching, results in very sensitive methods even with simple ultraviolet detection and may become the method of choice for routine analyses.

Human biliary metabolites of isotretinoin: identification, quantification, synthesis, and biological activity

1. The metabolites of isotretinoin (13-cis-retinoic acid, Accutane) were investigated in the bile of two patients with biliary T-tube drainage after administration of a single, oral, 80-mg dose of 14C-isotretinoin. Radioactivity measurements showed that the two patients excreted 22.7 and 17.1% of the dose in their bile in 4 days. 2. The two major drug-related components in the bile were identified as the glucuronide conjugates of 4-oxo-isotretinoin and 16-hydroxy-isotretinoin. Two minor components were identified as the glucuronide conjugates of isotretinoin and 18-hydroxy-isotretinoin. 3. H.p.l.c. analyses of Glusulase-treated bile samples indicated that the glucuronides of isotretinoin and the two major metabolites accounted for about 48% and 44% of the total radioactivity in the bile of the two patients. 4. Racemic 16-hydroxy-isotretinoin was synthesized and evaluated for its effect on human sebocytes in vitro. This metabolite and the other major metabolites of isotretinoin were less active than isotretinoin in inhibiting the proliferation of the sebocytes.