Tian X Q, Chen T C, Lu Z, Shao Q, Holick M F
Vitamin D, Skin, and Bone Research Laboratory, Boston University School of Medicine, Massachusetts 02118.
Endocrinology. 1994 Aug;135(2):655-61. doi: 10.1210/endo.135.2.8033813.
The cutaneous synthesis of vitamin D3 and the subsequent translocation of vitamin D3 into the circulation are two key steps in the vitamin D endocrine system. To study the kinetic aspects of cutaneous synthesis and translocation of vitamin D3, both in vitro and in vivo chicken models have been developed. To assess the capacity of chicken skin to generate vitamin D3, the concentrations of 7-dehydrocholesterol (7-DHC) in different skin areas were determined. It was found that the highest concentration of 7-DHC was in the leg skin (3524 +/- 937 ng cm-2), which was about 30 times greater than that in the back (120 +/- 62 ng cm-2). Whole body exposure of chickens to UV-B radiation (0.5 J cm-2) resulted in the production of previtamin D3 (preD3) in the skin of the legs and feet (43 +/- 7 and 54 +/- 17 ng cm-2, respectively), whereas no preD3 was detected in the back skin. In vitro, at 40 C, the forward (k1) and reverse (k2) rate constants of the preD3<-->vitamin D3 reaction in the leg skin were greatly increased compared to those in n-hexane (k1, 0.367 vs. 0.0369 h-1; k2, 0.042 vs. 0.0059 h-1). In vivo, the determined rate constants k1, k2, and k3 for the consecutive reactions preD3<-->vitamin D3-->vitamin D3 were 0.257, 0.034, and 0.114 h-1, respectively. To evaluate the circulating concentration of vitamin D3 in response to UV-B radiation, chicken legs were irradiated. The time course revealed a 4-fold increase in the circulating concentration of vitamin D3, with a peak about 30 h postradiation. No appreciable amount of preD3 could be detected in the circulation in the early hours after UV-B radiation, suggesting the existence of a process responsible for the specific translocation of vitamin D3 from the skin into the circulation.
维生素D3的皮肤合成以及随后维生素D3向循环系统的转运是维生素D内分泌系统中的两个关键步骤。为了研究维生素D3皮肤合成和转运的动力学方面，已经建立了体外和体内鸡模型。为了评估鸡皮肤产生维生素D3的能力，测定了不同皮肤区域中7-脱氢胆固醇（7-DHC）的浓度。发现腿部皮肤中7-DHC的浓度最高（3524±937 ng/cm²），约为背部（120±62 ng/cm²）的30倍。将鸡全身暴露于UV-B辐射（0.5 J/cm²）会导致腿部和脚部皮肤产生维生素D3原（preD3）（分别为43±7和54±17 ng/cm²），而背部皮肤中未检测到preD3。在体外，40℃时，腿部皮肤中preD3⇌维生素D3反应的正向（k1）和反向（k2）速率常数与正己烷中相比大幅增加（k1，0.367对0.0369 h⁻¹；k2，0.042对0.0059 h⁻¹）。在体内，连续反应preD3⇌维生素D3⇌维生素D3的测定速率常数k1、k2和k3分别为0.257、0.034和0.114 h⁻¹。为了评估响应UV-B辐射时维生素D3的循环浓度，对鸡腿进行了辐照。时间进程显示维生素D3的循环浓度增加了4倍，在辐射后约30小时达到峰值。在UV-B辐射后的早期小时内，循环中未检测到可观量的preD3，这表明存在一个负责维生素D3从皮肤特异性转运到循环中的过程。
