Comparative biochemical study on glucocorticoid metabolism in mammary gland and kidney

皮质类固醇激素的作用受11β-羟基类固醇脱氢酶(11β-HSD)的调节，目前已认识到11β-HSD的两种同工酶：11β-HSD-1是NADP依赖的酶，Km在微摩尔范围内。11β-HSD-2是一种依赖NAD的酶，Km在纳摩尔范围内。在肝脏中，11β-HSD-1占主导地位，将11-脱氢皮质酮转化为皮质酮。另一方面，在肾脏中，依赖NAD的11β-HSD-2氧化失活皮质酮，允许非选择性的盐皮质激素受体被盐皮质激素占据。本研究发现11β-HSD-2存在于哺乳动物乳腺和产蛋鸟生殖道等其他钠转运上皮细胞中。从日本鹌鹑肾脏匀浆和输卵管不同部位的匀浆中提取出11β-HSD-2，并在40℃和1 mM NAD存在或不存在的情况下与皮质酮孵育1小时，停止反应，用乙酸乙酯提取类固醇。使用在氯仿-甲醇(95：5)体系中发展的薄层层析法分离类固醇。通过测定皮质酮向11-脱氢皮质酮的转化率来测定11β-HSD-2的活性。在所研究的一些组织中，输卵管的壳腺和蛋白分泌区显示出较高的11β-HSD-2活性。在这些组织中，放射性皮质酮被完全代谢成11-脱氢皮质酮。另一方面，输卵管漏斗、峡部和阴道的活性较低，提示这些部位可能不是盐皮质激素的靶标。虽然蛋白的分泌和蛋壳的形成过程尚不清楚是由盐皮质激素介导的，但我们的发现表明这些组织可能是盐皮质激素的靶标。

Corticosteroid action is modulated by the enzyme 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) Two isozymes of 11 beta-HSD are now recognized : 11 beta-HSD-1 is NADP-dependent enzyme with a Km in the micromolar range. 11 beta-HSD-2 is NAD-dependent enzyme with a Km in the nanomolar range. In the liver, 11 beta-HSD-1 is predominant and converts 11-dehydrocorticosterone to corticosterone. On the other hand, in the kidney, the NAD-dependent 11 beta-HSD-2 oxidatively inactivates corticosterone, allowing the nonselective mineralocorticoid receptor to be occupied by mineralocorticoids. The present study was conducted to show that 11 beta-HSD-2 is present in other sodium-transporting epithelia such as those in the mammary gland of mammals and in the reproductive tract of egg-laying birds.The microsome fraction was prepared from homogenates of kidney and various parts of oviduct of Japanese quail, and incubated with traitiated corticosterone for 1 hr at 40 C in the presence or absence of 1 mM NAD.The reaction was stopped, and the steroid was extracted by ethyl acetate. Separation of steroids was performed using a thin-layr chromatography developed in a chloroform-methanol (95 : 5) system. Activity of 11 beta-HSD-2 was determined by measuring the conversion rate of corticosterone to 11-dehydrocorticosterone.Among a number of tissues studied, the shell gland and albumen secreting region of the oviduct showed high activity of 11 beta-HSD-2. In these tissue, radioactive corticosterone was exclusively metabolized to 11-dehydrocorticosterone. On the other hands, infundibulum and isthmus of the oviduct and the vagina contain low activity, indicationg that these parts might not be a target of the mineralocorticoids. Although the process of secretion of albumen and the formation of egg shell is not known to be mediated by mineralocorticoids, our findings indicate that these tissues might be a target of the mineralocorticoids.