Cytochrome P450 27C1 (P450 27C1) is a retinoid desaturase expressed in the skin that catalyzes the formation of 3,4-dehydroretinoids from all-trans retinoids. Within the skin, retinoids are important regulators of proliferation and differentiation. In vivo, retinoids are bound to cellular retinol-binding proteins (CRBPs) and cellular retinoic acid-binding proteins (CRABPs). Interaction with these binding proteins is a defining characteristic of physiologically relevant enzymes in retinoid metabolism. Previous studies that characterized the catalytic activity of human P450 27C1 utilized a reconstituted in vitro system with free retinoids. However, it was unknown whether P450 27C1 could directly interact with holo-retinoid-binding proteins to receive all-trans retinoid substrates. To assess this, steady-state kinetic assays were conducted with free all-trans retinoids and holo-CRBP-1, holo-CRABP-1, and holo-CRABP-2. For holo-CRBP-1 and holo-CRABP-2, the kcat/Km values either decreased 5-fold or were equal to the respective free retinoid values. The kcat/Km value for holo-CRABP-1, however, decreased ∼65-fold in comparison with reactions with free all-trans retinoic acid. These results suggest that P450 27C1 directly accepts all-trans retinol and retinaldehyde from CRBP-1 and all-trans retinoic acid from CRABP-2, but not from CRABP-1. A difference in substrate channeling between CRABP-1 and CRABP-2 was also supported by isotope dilution experiments. Analysis of retinoid transfer from holo-CRABPs to P450 27C1 suggests that the decrease in kcat observed in steady-state kinetic assays is due to retinoid transfer becoming rate-limiting in the P450 27C1 catalytic cycle. Overall, these results illustrate that, like the CYP26 enzymes involved in retinoic acid metabolism, P450 27C1 interacts with cellular retinoid-binding proteins.

中文翻译：

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细胞类视黄醇结合蛋白将类视黄醇转移到人细胞色素 P450 27C1 上进行去饱和。

细胞色素 P450 27C1 (P450 27C1) 是一种在皮肤中表达的类视黄醇去饱和酶，可催化全反式类视黄醇形成 3,4-脱氢类视黄醇。在皮肤内，类视色素是增殖和分化的重要调节剂。在体内，类视黄醇与细胞视黄醇结合蛋白 (CRBP) 和细胞视黄酸结合蛋白 (CRABP) 结合。与这些结合蛋白的相互作用是维甲酸代谢中生理相关酶的定义特征。先前表征人 P450 27C1 催化活性的研究利用了含有游离类视色素的重组体外系统。然而，尚不清楚 P450 27C1 是否可以直接与全维甲酸结合蛋白相互作用以接收全反式维甲酸底物。为了评估这一点，稳态动力学测定是用游离的全反式维甲酸和holo-CRBP-1、holo-CRABP-1和holo-CRABP-2进行的。对于holo-CRBP-1 和holo-CRABP-2，kcat/Km 值要么下降了5 倍，要么等于各自的游离类视黄醇值。然而，与使用游离全反式视黄酸的反应相比，holo-CRABP-1 的 kcat/Km 值降低了约 65 倍。这些结果表明 P450 27C1 直接接受来自 CRBP-1 的全反式视黄醇和视黄醛以及来自 CRABP-2 的全反式视黄酸，但不接受来自 CRABP-1 的全反式视黄酸。同位素稀释实验也支持 CRABP-1 和 CRABP-2 之间底物通道的差异。对从全息 CRABP 到 P450 27C1 的类视黄醇转移的分析表明，在稳态动力学分析中观察到的 kcat 降低是由于类视黄醇转移成为 P450 27C1 催化循环中的速率限制。总体而言，这些结果表明，与参与视黄酸代谢的 CYP26 酶一样，P450 27C1 与细胞维甲酸结合蛋白相互作用。