Study of 1,25-dihydroxyvitamin D3 induced alterations in the metabolism of [3H]25-hydroxyvitamin D3 using isolated perfused kidneys from D-sufficient rats

Vitamin D metabolite profiles in moderate renal insufficiency of childhood

In a prior study, we reported vitamin D metabolite profiles in the plasma of healthy children. In view of these findings, we investigated these same profiles in children with moderate renal insufficiency. Specimens were obtained in both summer and winter in untreated patients, and before and after treatment for up to 1 year, with either 25(OH)D3 or 1,25(OH)2D3. Mean pretreatment 1,25(OH)2D levels were normal. Levels of 25(OH)D3 were also normal and continued to vary with season. Mean pretreatment 24,25(OH)2D3 levels were significantly lower in patients and, interestingly, did not show the normal seasonal variation. Treatment with 25(OH)D3 resulted in consistent and sustained rises in 25(OH)D3 and 24,25(OH)2D3 levels, but no increase in 1,25(OH)2D levels. After 1,25(OH)2D3 treatment, 25(OH)D3 levels were unchanged but still showed seasonal variation in some patients, suggesting a lack of feedback control by 1,25(OH)2D3. Levels of 24,25(OH)2D3 were not significantly different from baseline values. Levels of 1,25(OH)2D increased initially then dropped to pretreatment levels or lower. Normal 1,25(OH)2D levels and reduced 24,25(OH)2D3 levels with the loss of seasonal variation suggests in our patients that the kidney was able to maintain 1,25(OH)2D levels at the expense of 24,25(OH)2D3 levels, presumably to preserve calcium homeostasis.

Isolation and identification of 1,24,25-trihydroxyvitamin D2, 1,24,25,28-tetrahydroxyvitamin D2, and 1,24,25,26-tetrahydroxyvitamin D2: new metabolites of 1,25-dihydroxyvitamin D2 produced in rat kidney

Three new metabolites of vitamin D2 were produced in vitro by perfusing isolated rat kidneys with 1,25-dihydroxyvitamin D2. They were isolated and purified from the kidney perfusate by the techniques of methanol-methylene chloride lipid extraction and high-performance liquid chromatography. By means of ultraviolet absorption spectrophotometry, mass spectrometry, and specific chemical reactions, the metabolites were identified as 1,24,25-trihydroxyvitamin D2, 1,24,25,28-tetrahydroxyvitamin D2, and 1,24,25,26-tetrahydroxyvitamin D2. Both 1,24,25,28-tetrahydroxyvitamin D2 and 1,24,25,26-tetrahydroxyvitamin D2 were also produced when a kidney was perfused with 1,24,25-trihydroxyvitamin D2. Thus, it becomes clear that 1,25-dihydroxyvitamin D2 is first hydroxylated at C-24 to form 1,24,25-trihydroxyvitamin D2, which is then further hydroxylated at C-28 and C-26 to form 1,24,25,28-tetrahydroxyvitamin D2 and 1,24,25,26-tetrahydroxyvitamin D2, respectively. From several recent studies, it has been well established that 1,25-dihydroxyvitamin D3 is converted into various further metabolites in the kidney as a result of chemical reactions such as C-23, C-24, and C-26 hydroxylations, C-24 ketonization, and C-23:C-26 lactonization. From our study it is obvious that 1,25-dihydroxyvitamin D2 does not undergo all of the aforementioned chemical reactions except C-24 and C-26 hydroxylations. Also, our study indicates that C-28 hydroxylation plays a significant role in the further metabolism of 1,25-dihydroxyvitamin D2. Thus, for the first time, we describe a novel further metabolic pathway for 1,25-dihydroxyvitamin D2 in a mammalian kidney.

23,25-Dihydroxy-24-oxovitamin D3: a metabolite of vitamin D3 made in the kidney

Kidney homogenates of rats produced a new metabolite of 25-hydroxyvitamin D3 which has been isolated in pure form after five column chromatographic steps. It was identified as 23,25-dihydroxy-24-oxovitamin D3 by means of ultraviolet and infrared absorption spectrophotometry, mass spectrometry, and proton nuclear magnetic resonance spectrometry. The stereochemistry at the C-23 position is as yet unknown. 25-Hydroxy-24-oxovitamin D3, which also has been isolated in pure form from this system, was found to be the precursor of the new metabolite in vitro. The production of the new metabolite was induced by two different methods: (a) perfusion of the kidneys with 1,25-dihydroxyvitamin D3 contained in the perfusate and (b) injection of 1,25-dihydroxyvitamin D3 in the intact animal. 23,25-Dihydroxy-24-oxovitamin D3 was not biologically active in an assay for intestinal calcium transport and bone calcium mobilization in the vitamin D deficient chick at a dose level of 5.3 nmol. A metabolic pathway is proposed to describe the results; it leads from 25-hydroxyvitamin D3 leads to 24(R),25-dihydroxyvitamin D3 leads to 25-hydroxy-24-oxovitamin D3 leads to 23,25-dihydroxy-24-oxovitamin D3.