BIOLOGICALLY ACTIVE FORMS OF VITAMIN D3 IN KIDNEY AND INTESTINE

Vitamin D metabolite profiling using liquid chromatography-tandem mass spectrometry (LC-MS/MS)

Liquid chromatography tandem mass spectrometry (LC-MS/MS) has emerged as the latest technology to be used to assay the metabolites of vitamin D. The method uses molecular mass as a detection technique after straightforward extraction and chromatography steps. LC-MS/MS assay provides a level of accuracy and reproducibility not seen before with other methods and is beginning to rival antibody-based methods in terms of sensitivity and convenience. Methods for detection of underivatized and DMEQ-TAD derivatized vitamin D metabolites are evaluated. Sensitivity is improved by 10-100 fold with derivatization and allows for the simultaneous assay of multiple vitamin D metabolites, a process termed vitamin D metabolite profiling. Clinical and research applications of vitamin D metabolite profiling are discussed.

Is there more to learn about functional vitamin D metabolism?

The state of information on the enzymes responsible for the conversion of vitamin D3 to 1α,25-dhydroxyvitamin D3 (1,25-(OH)2D3), the metabolic active form responsible for the well-known function of vitamin D on calcium metabolism and bone mineralization has been briefly reviewed. There remains an unidentified enzyme responsible for 25% of the 25-hydroxylation of vitamin D3, while 75% of serum 25-hydroxyvitamin D3 (25-OH-D3) arises from CYP2R1. The well-established suppression of multiple sclerosis (MS) by sunlight has been confirmed using the mouse model, experimental autoimmune encephalomyelitis (EAE). This suppression results from a narrow band of ultraviolet light (300-315nm) that does not increase serum 25-OH-D3. Thus, UV light suppresses EAE by a mechanism not involving vitamin D. Vitamin D deficiency unexpectedly suppresses the development of EAE. Further, vitamin D receptor knockout in susceptible mice also prevents the development of EAE. On the other hand, deletion of CYP2R1 and the 1α-hydroxylase, CYP27B1, does not impair the development of EAE. Thus, either vitamin D itself or a heretofore-unknown metabolite is needed for the development of a component of the immune system necessary for development of EAE. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

History of the discovery of vitamin D and its active metabolites

Before the twentieth century, it was not possible to describe the essentials of a diet that could support life, growth and reproduction of higher animals. The discovery of vitamin A by McCollum and Davis in 1913 ushered in the era of accessory food substances culminating in the achievement of that goal. It included the discovery of vitamin D and its production in skin caused by ultraviolet light. This was followed by a description of its actions at the physiological level that resulted in a healthy skeleton and beyond. To carry out these functions, vitamin D is converted to a hormone that acts through a nuclear receptor. The findings leading to this concept and their importance to biology and medicine are presented.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Differential absorption of vitamin D3 and 1,25-dihydroxyvitamin D3 by intestinal jejunal cells of the rat

An intestinal perfusion technique is reported for the study of the differential absorption of vitamin D3 and its active metabolite, 1,25-dihydroxyvitamin D3, through intact jejunal segments of rats. Samples of introduced and collected perfusates, intestinal homogenates, and portal blood were assayed for [14C]vitamin D3 or [3H]1 alpha,25-dihydroxyvitamin D3 content at specified time intervals in control rats and in rats injected ip with cycloheximide (3 mg/kg body weight). Vitamin D3 uptake from the perfusates in cycloheximide-treated groups did not differ from controls. However, an approximately 2-fold increase of vitamin D3 retention in the perfused intestinal segments was observed after cycloheximide treatment. A 0.25-fold decrease was observed in the uptake of 1,25-dihydroxyvitamin D3 from the perfusates after cycloheximide treatment, and an approximately 2.5-fold increase in its intestinal retention was noted. An increase in the active metabolite concentration was observed in the portal venous system 75 min after initiation of perfusion, with no detectable amounts being recorded prior to the first hour. The results suggest that intracellular binding proteins may be involved in the transport of labeled vitamin D3 and labeled 1,25-dihydroxyvitamin D3 through rat enterocytes. Furthermore, vitamin D3 may have been more readily channeled through an esterification process than 1,25-dihydroxyvitamin D3 prior to their appearance in the portal venous system.

Sequestration and microsomal C-25 hydroxylation of [3H]-vitamin D3 by the rat liver

A study of the vitamin D3 (D3) 25-hydroxylase was undertaken in an in vivo-in vitro model. [3H]-D3 (0.7, 1.0, 10, or 100 nmol/100 g of body weight) was injected into the portal vein and the liver was excised 18 seconds later. The liver homogenate was then submitted to differential centrifugation and the amount of [3H]-D3 incorporated in the subcellular fractions was evaluated. The microsomal fraction was also incubated in vitro and the appearance of [3H]-25-hydroxyvitamin D3 [25(OH)D3] was determined by high performance liquid chromatography (HPLC). Results showed that the fractional liver [3H]-D3 uptake varied between 37 percent and 48 percent of the dose injected. The intracellular distribution of [3H]-D3 showed that most of the vitamin was incorporated into the microsomal fraction (45% to 50% of the intracellular [3H]-D3) except at the highest dose of [3H]-D3 where the cytosolic fraction contained the highest amount (56.4%) of the incorporated vitamin. Mathematical analysis of the intracellular [3H]-D3 distribution showed that the microsomal fraction was the only subcellular fraction that was found to incorporate [3H]-D3 in relation to the total liver uptake of the vitamin. The apparent Michaelis-Menten kinetics of the [3H]-D3-25-hydroxylase showed that with substrate concentration of up to 88.5 nM, the apparent Km and Vmax were 28.2 nM and 25.8 fentomoles (fmol) X min-1 X mg microsomal pro-1, respectively, but the reaction lost considerable efficiency with higher substrate concentrations. With the in vivo-in vitro model used, the cytosolic fraction was not essential for the optimal C-25 hydroxylation of D3. These results show that the endoplasmic reticulum of rat hepatocytes possess a high capacity for D3 incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)

Milk fever in dairy cows. VIII. Effect of injected vitamin D3 and calcium and phosphorus intake on incidence

In a field trial, effects of prepartal intermuscular injection of 10 million units of vitamin D3 on incidence of milk fever were examined both in relation to intake of calcium and phosphorous during the dry period and previous history of milk fever. Based upon intake of calcium and phosphorus cooperating herds were grouped as feeding: 1) greater than .53% of the total ration dry matter as calcium and greater than .28% as phosphorus: 2) less than .47% as calcium and greater than .28% as phosphorus; 3) greater than .47% to less than .53% as calcium and greater than .22% to less than .28% as phosphorus. Injections of vitamin D3 given approximately 1 wk prepartum reduced incidence of milk fever in cows with previous history of milk fever in all three groups but had no effect in cows with no previous milk fever. Incidence of milk fever was lower in group 3 than for cows of groups 1 and 2 with previous milk fever and than for cows of group 1 with no previous history. The results indicate that careful control of calcium and phosphorus intake during the dry period at .5% calcium and .25% phosphorus of the dry matter of the total ration will limit milk fever incidence to about 10%. Injections of vitamin D3 as described will reduce inicidence of milk fever further in cows with previous milk fever but not in cows with no previous milk fever.

[Determination of vitamin D3 and its isomers in D3 resins and in mixtures with vitamin A acetate (author's transl)]

Vitamin D3 and its isomers, prepared from 7-dehydrocholesterol by photochemical reaction, can be separated and quantitatively determined by high-pressure liquid chromatography on aluminium oxide using chloroform as the eluting agent. The determination of small amounts of vitamin D3 in an excess of vitamin A acetate can be achieved by the same method.

Studies on the renal uptake of vitamin D3 in the mouse and the quail

The renal uptake of labelled vitamin D3 was studied in the mouse and the quail. Upon the administration of labelled vitamin D3 autoradiographic experiments showed a specific accumulation of radioactivity in the proximal tubuli of the mouse kidney. This was still obvious 18 days after the administration. In the quail, on the other hand, the uptake in the kidney did not exceed the level of the blood. In the mouse there was a slow increase in the amount of steriod in the kidney after the injection of vitamin D3, a maximum being reached 24 hours after the administration. The amount of steroid which accumulated in the kidney was largely proportional to the injected dose of the vitamin-from doses at 4.8 ng to 4.8 mug. Column chromatography showed that most of the renal vitamin D3, was present in a non-metabolized form. Cellular fractionation showed that most radioactivity in the kidney was present in the mitochondrial and microsomal fractions. Upon sonication of the fractions most radioactivity was still bound to these particles.

Inhibition of the metabolism of 25-hydroxycholecalciferol by actinomycin D and cycloheximide

Actinomycin D (or cycloheximide) administered prior to radioactive 25-hydroxycholecalciferol blocks the metabolism of 25-hydroxycholecalciferol to polar metabolites that accumulate in intestinal tissue, while it does not prevent the 25-hydroxylation of vitamin D(3) in the liver. Actinomycin D given after radioactive 25-hydroxycholecalciferol does not inhibit 25-hydroxycholecalciferol metabolism. These results indicate that 25-hydroxycholecalciferol must interact with the nuclei of cells to bring about the production of an enzyme(s) that converts it to its polar metabolites.

"Activation" of vitamin D by the liver

Isolation of the liver from the circulation of rats eliminates almost completely their ability to convert [1,2]-(3)H vitamin D(3) into its biologically active metabolite, 25-hydroxycholecalciferol, as well as certain other metabolites. It is concluded that the liver is the major if not the only physiologic site of hydroxylation of vitamin D(3) (cholecalciferol) into 25-hydroxycholecalciferol. The osteodystrophy and the higher requirements for vitamin D observed in hepatic insufficiencies may be due to an inability of the liver to transform vitamin D into its metabolically active form.

Metabolism of vitamin D. A new cholecalciferol metabolite, involving loss of hydrogen at C-1, in chick intestinal nuclei

1. A comparison was made of the nature and intestinal intracellular distribution of the metabolites formed in vitamin D-deficient chicks from [4-(14)C]cholecalciferol and [1-(3)H]cholecalciferol. 2. The simultaneous administration of the two radioactive substances showed the presence in blood, liver, intestine, kidney and bone of cholecalciferol, its ester, 25-hydroxycholecalciferol and a further metabolite of cholecalciferol more polar than 25-hydroxycholecalciferol. The (3)H/(14)C ratios in these four radioactive components were the same as that of the dosed material (4.7:1) with the exception of the most polar material. The (3)H/(14)C ratio was lower in the fourth, most polar, metabolite (0.4:1-1.8:1) in all tissues examined, with the exception of blood. 3. In the chick intestine the polar metabolite accounted for almost 70% of the radioactivity in this tissue after a dose of 0.5mug. of [4-(14)C,1-(3)H]cholecalciferol. This polar metabolite from the intestine also had the lowest (3)H/(14)C ratio of all the tissues. It appears that in the chick intestine the polar metabolite reaches a maximum concentration of 1ng./g. of tissue, above which it cannot be increased irrespective of the dose of the vitamin. 4. The intestinal intracellular organelle with the highest concentration of (14)C radioactivity is the nucleus, and this radioactivity is almost entirely due to the polar metabolite with the lowered (3)H/(14)C ratio, in this case <0.2:1. It appears to be further localized in the chromatin of the nuclei. However, about half of the polar metabolite in the intestine is extranuclear. 5. Double-labelled 25-hydroxycholecalciferol was prepared and after its administration to vitamin D-deficient chicks the polar metabolite with the lowered (3)H/(14)C ratio was detected in liver, kidney, intestine, bone, muscle and heart. 6. None of the polar metabolite with the lowered (3)H/(14)C ratio was detected 16hr. after dosing with either the double-labelled vitamin or the double-labelled 25-hydroxycholecalciferol in blood and adipose tissue of vitamin D-deficient chicks, nor in the intestine, liver and kidney of supplemented birds. 7. The reasons for this loss of (3)H relative to (14)C are discussed in relation to possible chemical structures of this new polar metabolite.

The role of the liver in the metabolism of vitamin D

The metabolism of vitamin D(3) has been studied after intravenous injection of 10 IU of [1,2-(3)H]-vitamin D(3) to vitamin D-deficient rats. The disappearance of the radioactivity from the plasma follows a very peculiar pattern characterized by an early rapid disappearance followed by a rebound of radioactivity before assuming still a third rather slow disappearance rate. The "rebound" phenomenon is concomitant with a rapid release of the radioactivity from the liver and is accounted for by the appearance of 25-hydroxycholecalciferol and other metabolites in the blood. It is postulated that the liver is the major site of transformation of vitamin D(3) into 25-hydroxycholecalciferol.

The metabolism and biological activity of esterified vitamin D in the rat

1. [I-3H]Cholecalciferol and [I-3H]cholecalciferol palmitate in amounts equivalent to 2 μg cholecalciferol were injected into rachitic rats as aqueous preparations (intravenously) or in arachis oil (intramuscularly).

2. The radioactive faecal metabolites collected for up to 17 days were fractionated according to their polarity. The same pattern of excreted radioactivity was seen with intravenous and intramuscular cholecalciferol and intravenous cholecalciferol palmitate. Intramuscular chole-calciferol palmitate produced a higher proportion of the most polar metabolites in faeces.

3. Parenterally administered cholecalciferol palmitate in a rickets-healing biological assay had activity equivalent to unesterified cholecalciferol.

4. Vitamin D ester synthesized in vivo is considered to be eventually metabolized as vita- min D alcohol and to be available to vitamin D-requiring processes.

Studies in the absorption and metabolism of vitamin D after gastric surgery

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Investigations on vitamin D esters synthesized rats. Detection and identification

1. Vitamin D-deficient rachitic rats were given [1-(3)H]cholecalciferol by gastric intubation. After 24hr., diethyl ether extracts of liver and kidney contained 5-11% and 4.5-20% respectively of total vitamin D apparently esterified with long-chain fatty acids. 2. A two-dimensional thin-layer chromatographic technique was devised that completely separated seven synthetic vitamin D esters according to the chain length and number of double bonds in the fatty acid component. When the ;vitamin D ester' fraction from liver or kidney was co-chromatographed with the standard esters, radioactivity appeared mainly in vitamin D palmitate, stearate, oleate and linoleate regions. The proportion of radioactivity in the saturated fatty acid esters was higher in kidney than in liver. 3. The same percentage of tissue vitamin D in the esterified form was found at each of two dosages of vitamin D. 4. The possible specificity of a vitamin D esterification mechanism is discussed.

Metabolism of vitamin D3-3H in vitamin D-resistant rickets and familial hypophosphatemia

The fate of an intravenous dose of tritiated vitamin D(3) was studied in seven normal subjects, four children with vitamin D-resistant rickets, and four adults with a familial history of vitamin D-resistant rickets and persistent hypophosphatemia. An abnormal metabolism of vitamin D in vitamin D-resistant rickets was defined and characterized by a decrease in the plasma fractional turnover rate, a marked increase in plasma water-soluble metabolites, and a relative decrease in the conversion of vitamin D to a polar, biologically active metabolite. Alterations in vitamin D metabolism in the adults with persistent hypophosphatemia were similar but less severe than those of affected children with vitamin D-resistant rickets. It is tentatively concluded that the abnormalities in vitamin D metabolism documented in patients with vitamin D-resistant rickets and familial hypophosphatemia may account for the observed osseous and biochemical changes.

Metabolism of vitamin D3-3H in human subjects: distribution in blood, bile, feces, and urine

Vitamin D(3)-(3)H has been administered intravenously to seven normal subjects, three patients with biliary fistulas, and four patients with cirrhosis. Plasma D(3)-(3)H half-times normally ranged from 20 to 30 hours. in vivo evidence that a metabolic transformation of vitamin D occurs was obtained, and a polar biologically active vitamin D metabolite was isolated from plasma. Urinary radioactivity averaged 2.4% of the administered dose for the 48-hour period after infusion, and all the excreted radioactivity represented chemically altered metabolites of vitamin D. The metabolites in urine were mainly water-soluble, with 26% in conjugated form. From 3 to 6% of the injected radioactivity was excreted in the bile of subjects with T-tube drainage and 5% in the feces of patients having no T-tube. The pattern of fecal and biliary radioactivity suggested that the passage of vitamin D and its metabolites from bile into the intestine represents an essential stage for the fecal excretion of vitamin D metabolites in man. Abnormally slow plasma disappearance of vitamin D(3)-(3)H in patients with cirrhosis was associated with a significant decrease in the quantity and rate of glucuronide metabolite excretion in the urine.