William C. Rose lectureship in biochemistry and nutrition. Some new concepts emanating from a study of the metabolism and function of Vitamin D

The Diverse Roles of 17β-Estradiol in Non-Gonadal Tissues and Its Consequential Impact on Reproduction in Laying and Broiler Breeder Hens

Estradiol-17β (E2) has long been studied as the primary estrogen involved in sexual maturation of hens. Due to the oviparous nature of avian species, ovarian production of E2 has been indicated as the key steroid responsible for activating the formation of the eggshell and internal egg components in hens. This involves the integration and coordination between ovarian follicular development, liver metabolism and bone physiology to produce the follicle, yolk and albumen, and shell, respectively. However, the ability of E2 to be synthesized by non-gonadal tissues such as the skin, heart, muscle, liver, brain, adipose tissue, pancreas, and adrenal glands demonstrates the capability of this hormone to influence a variety of physiological processes. Thus, in this review, we intend to re-establish the role of E2 within these tissues and identify direct and indirect integration between the control of reproduction, metabolism, and bone physiology. Specifically, the sources of E2 and its activity in these tissues via the estrogen receptors (ERα, ERβ, GPR30) is described. This is followed by an update on the role of E2 during sexual differentiation of the embryo and maturation of the hen. We then also consider the implications of the recent discovery of additional E2 elevations during an extended laying cycle. Next, the specific roles of E2 in yolk formation and skeletal development are outlined. Finally, the consequences of altered E2 production in mature hens and the associated disorders are discussed. While these areas of study have been previously independently considered, this comprehensive review intends to highlight the critical roles played by E2 to alter and coordinate physiological processes in preparation for the laying cycle.

Effects of sunlight on behavior and 25-hydroxyvitamin D levels in two species of Old World fruit bats

It has long been accepted that most vertebrate animals meet their vitamin D requirements from exposure of skin to UV-B (UV-B) radiation. Many factors affect this endogenous synthesis of vitamin D, including season, latitude, time of day, age, presence of hair, and degree of skin pigmentation. Most bats roost in dark places by day and forage at night, and thus have little or no potential for sunlight exposure. Notwithstanding, some tropical species are diurnal and are known to roost in the canopy of trees where they may be exposed to sunlight for up to 12 h each day. In this study, two species of captive tropical bats (both species are active at night but one, Rousettus aegyptiacus, roosts in caves, tombs, and buildings, whereas the other, Pteropus hypomelanus, roosts in trees) were evaluated for their ability to endogenously synthesize vitamin D. Following timed periods of sunlight exposure, blood plasma was analyzed using a competitive protein binding assay (CPBA) to determine concentrations of 25-hydroxyvitamin D [25(OH)D], the major circulating vitamin D metabolite. The ability to photoconvert provitamin D (7-dehydrocholesterol, 7-DHC) in the sub-tropical winter was determined using sunlight exposed borosilicate samples of 7-DHC in hourly increments. Finally, both species were evaluated in their preference for a roost site by the release of individuals into sunlight or shade in timed trials.

OUR RESULTS SUPPORT THE HYPOTHESES

(1) when exposed to natural sunlight, both species exhibited an ability to endogenously synthesize vitamin D, although significant differences were found between the two, (2) photoconversion of 7-DHC to previtamin D3 is possible during the mid-day hours of a sub-tropical winter day and (3) captive, cave roosting R. aegyptiacus will choose shaded roost sites while captive P. hypomelanus will show no preference for either shade or sun.

Synthesis and determination of configuration of natural 25-hydroxyvitamin D(3) 26,23-lactone

The four stereoisomers of 25-hydroxyvitamin D(3) 26,23-lactone were synthesized by a stereoselective lactonization method. Natural 25-hydroxyvitamin D(3) 26,23-lactone was produced from 25-hydroxy-[3alpha-(3)H]vitamin D(3) by in vitro incubation of kidney homogenate prepared from vitamin D-supplemented chickens or was isolated from the serum of rats given 1,25-dihydroxyvitamin D(3) and 25-hydroxy-[3alpha-(3)H]vitamin D(3). The four synthetic isomers and the naturally produced 25-hydroxyvitamin D(3) 26,23-lactone were subjected to high-performance liquid chromatography in a system capable of separating the four isomers. The natural lactone comigrated with synthetic (23S,25R)-25-hydroxyvitamin D(3) 26,23-lactone, establishing it as the natural vitamin D(3) metabolite.

Nutrient content of liquid milk

A survey was undertaken to update and extend available information on the vitamin content of pasteurized milk as produced at processing dairies in mainland UK and to investigate regional, seasonal and breed effects. The concentration of total retinol in milk from non-Channel Island (NCI) breeds averaged 61·9 βg/100g in summer and 41·2 βg/100g in winter. Concentrations of β-carotene were 31·5 and 10·5 βg/100g in summer and winter respectively. Concentrations of retinol in milk from Channel Island (CI) breeds were similar, but concentrations of β-carotene were on average 3 times higher. The concentration of vitamin D3in milk from NCI breeds was 0·033 βg/100g in summer and 0·026 βg/100g in winter. There was no marked seasonal variation in the mean concentration of total vitamin C (14·5 βg/ml). Values for the concentration of B vitamins (βg/ml) were: folic acid 0·060, vitamin B120·0042, riboflavin 1·78, nicotinic acid 0·71, pantothenic acid 3·60, biotin 0·020, thiamin 0·46 and vitamin B60·61. Seasonal variation in the concentration was most marked for folic acid (c.v. 17·4%) and to a lesser extent for vitamin B12(c.v. 10·3%). The only breed differences in the B vitamin content were for riboflavin and folic acid, the mean values obtained for milk from CI breeds being respectively 20 and 10 % higher than those from NCI breeds.

Calcium in the diet: food sources, recommended intakes, and nutritional bioavailability

Calcium nutritional status among some groups in the United States is suboptimal when judged by calcium intakes and the high prevalence of osteoporosis. Unfortunately, however, it is not clear that increases in calcium intake will have a significant impact on osteoporosis or other chronic diseases that have been linked to calcium nutriture. There is still considerable controversy surrounding the issue of calcium RDAs. The body's ability to adapt to varying levels of calcium intakes, the lack of sensitive indicators of calcium status, and the complexity and slow progression of chronic diseases such as osteoporosis make it very difficult to establish the role of diet in this regard. Great progress has been made in the study of calcium absorption. Much is known about the mechanisms involved in calcium absorption and its regulation. Thus, a rapidly advancing field and further developments will be invaluable to our understanding of the role of diet in calcium nutrition. Calcium bioavailability is affected by diet composition and the chemical form of calcium in foods. The calcium in dairy products is readily absorbed in the intestine. Lactose enhances calcium absorption efficiency under some conditions. Components of plants such as fiber, phytate, and oxalic acid may depress calcium absorption. High intakes of protein increase urinary losses of calcium but this effect may be partially offset by the phosphate association with most high-protein foods. Calcium absorption from salts used in supplement tablets is generally good. Absorption from salts such as calcium carbonate which require acid for dissolution may be poor in persons with achlorhydria unless the tablets are consumed with a meal. The practical significance of factors that may alter calcium bioavailability in normal mixed diets is difficult to assess. It may be a significant factor when calcium intakes are marginal or when absorption by the active transport, vitamin D-dependent process is impaired or not fully developed, i.e., it may be significant when vitamin D status is poor, in the elderly, and in young infants.

Metabolic bone disease in asymptomatic men after partial gastrectomy with Billroth II anastomosis

We sought to determine whether gastric surgery might be associated with metabolic bone disease in a well-characterized population, and if so to explore its etiology. Sixteen asymptomatic middle-aged men who had had partial gastrectomy with Billroth II anastomosis but no other risk factors for metabolic bone disease were compared with unoperated healthy controls. Studies included a dietary survey, biochemical tests of bone and mineral metabolism, radiographs of the spine, determinations of bone mineral content, and bone histomorphometry. The gastric surgery subjects exhibited frequent vertebral fractures and an unusual constellation of bone abnormalities characterized by decreased bone mineral content and hyperosteoidosis without evidence of osteomalacia. Although serum immunoreactive parathyroid hormone and 25-hydroxyvitamin D levels were not different, 1,25-dihydroxyvitamin D levels were significantly higher (p = 0.037), and 24,25-dihydroxyvitamin D levels were significantly lower (p less than 0.0001) in subjects than in controls. Partial gastrectomy with Billroth II anastomosis may be associated with asymptomatic but clinically important metabolic bone disease. The pathophysiology is uncertain, but appears to involve alterations in vitamin D metabolism.

Developmental changes in responsiveness to vitamin D metabolites

We have demonstrated that epiphyseal chondroblasts contain specific receptors for 24R,25-dihydroxy vitamin D3(24,25(OH)2D3) while diaphyseal osteoblasts contain specific receptors for 1 alpha 25-dihydroxy vitamin D3(1,25(OH)2D3). Both metabolites induce DNA synthesis and creatine kinase (CKBB) activity. We have also found that the responsiveness of rat kidney to these metabolites changes during development. In embryonic and early postnatal stages, the kidney responds to 24,25(OH)2D3, later to both 24,25(OH)2D3 and 1,25(OH)2D3, and the mature kidney only to 1,25(OH)2D3. These responses correlate with changes in the specific receptors present in the kidney. Furthermore, we have compared developmental changes in skeletal (epiphysis, diaphysis and mandibular condyle) and non-skeletal (kidney, cerebellum, cerebrum, liver and pituitary) tissue in both rat (a postnatal developer) and rabbit (a perinatal developer). Epiphyseal or diaphyseal chondroblasts at any stage of development were predominantly responsive to 24,25(OH)2D3, whereas osteoblasts were responsive to 1,25(OH)2D3. In contrast, condylar chondroblasts, kidney, cerebellum and pituitary responded to 24,25(OH)2D3 during early development and subsequently developed responsiveness to 1,25(OH)2D3. Using primary cell cultures from kidneys at different stages of maturation, we showed the same developmental pattern as in vivo. Chronic treatment of the cells with 24,25(OH)2D3, but not 1,25(OH)2D3, caused precocious development of responsiveness to 1,25(OH)2D3 in culture. We suggest that 24,25(OH)2D3 acts as a maturation factor, during early development in kidney, and probably in other tissues, possibly by induction of receptor to 1,25(OH)2D3, accompanied by down-regulation of its own receptor.

Assay and properties of rat yolk sac 25-hydroxyvitamin D3 1 alpha-hydroxylase

An in vitro assay has been developed for the rat yolk sac 25-hydroxyvitamin D3 1 alpha-hydroxylase (1 alpha-hydroxylase). The subcellular location and some properties of the enzyme are described. 1,25-Dihydroxyvitamin D3 produced from incubations of yolk sac homogenates was extracted, purified by Sephadex LH-20 chromatography and straight- and reverse-phase high-performance liquid chromatography (HPLC), and measured by a competitive binding assay using chick intestinal receptor. The reaction is linear with time for up to 45 min at a substrate concentration of 80 microM and 4-6 mg/mL microsomal protein. The enzyme, located in the microsomes, requires molecular oxygen and NADPH. Metyrapone (1 X 10(-3) M) was found to inhibit 1-hydroxylation, but a 90% carbon monoxide-10% oxygen atmosphere did not, leaving open the question of involvement of cytochrome P-450. Diphenyl-p-phenylenediamine, a lipid peroxidase inhibitor, inhibited 1 alpha-hydroxylation.

Low serum 25-hydroxyvitamin D in hereditary hemochromatosis: relation to iron status

Under normal conditions, vitamin D absorbed from the diet or synthesized in the skin is transported to the liver where it undergoes hydroxylation. The purpose of this study was to determine whether excess hepatic iron affects this process and the subsequent production of 1,25-dihydroxyvitamin D (1,25-[OH]2D) in the kidney. Mean serum 25-hydroxyvitamin D (25-OHD) concentrations in untreated hereditary hemochromatosis were 13 +/- 6 (SD) in 9 patients with cirrhosis, 13 +/- 6 in 5 patients with hepatic fibrosis, and 22 +/- 6 in 10 patients with normal hepatic architecture aside from siderosis and were significantly lower than the levels found in 24 controls matched for age, sex, and season, p less than 0.05. The mean serum 25-OHD levels in the two groups with hemochromatosis and hepatic damage were significantly lower than the value in the group with normal hepatic architecture, p less than 0.05. Serum 25-OHD levels in individual patients were inversely related to the size of body iron stores as measured by exchangeable body iron, r = -0.64, or serum ferritin, r = -0.47, p less than 0.05. In 15 patients removal of excess body iron by venesection therapy produced a significant increase in the mean serum 25-OHD from 20 ng/ml to 30 ng/ml, p less than 0.05. In contrast, mean serum 1,25-[OH]2D levels were similar in iron-loaded and control subjects, indicating that the regulation of this metabolite was intact in patients with hemochromatosis. The results reveal that the low serum 25-OHD concentration in patients with hemochromatosis is directly related to the extent of iron loading and it is improved by venesection therapy.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Autoradiographic localization of 24R,25-dihydroxyvitamin D3 in epiphyseal cartilage

There is emerging evidence for specific binding sites and biologic action for 24,25(OH)2D3 in the epiphyseal cartilage. The present study was undertaken to identify the target cells of 24,25(OH)2D3 in the epiphyses of rat bone using an autoradiographic technique. Pieces of epiphyseal cartilage obtained from 4-day-old vitamin D-deficient rats were incubated for 15 or 60 min with [3H]-24,25(OH)2D3 in the presence or absence of 100-fold excess of 25(OH)D3, 1,25(OH)2D3, or 24R,25(OH)2D3. The pieces were prepared for autoradiographic study by a new modified fixation method. Cytoplasmic and nuclear concentration of radioactivity was observed in all cell layers of the epiphyseal cartilage except for the hypertrophic cartilage zone. The highest concentration of radioactivity was seen in the proliferating chondroblasts of the columnar zone. After 15 min of incubation the radioactivity was seen mainly in the cell membrane and cytoplasm, whereas at 60 min radioactivity was also prominent in the nuclei. The competition with excess of cold metabolites revealed that only 24R,25(OH)2D3 caused a significant decrease in cytoplasmic and nuclear radioactivity. These data support the biochemical studies showing that the epiphyseal cartilage is a target tissue for 24,25(OH)2D3.

Vitamin D metabolites in adolescents and young adults with cystic fibrosis: effects of sun and season

To assess mineral metabolism in patients with cystic fibrosis and to study the effects of season and sunlight exposure on generation of vitamin D metabolites, we quantified serum levels of calcidiol and calcitriol, other measures of bone metabolism, and radiographic bone mass in 20 adolescents and young adults with CF and 20 age-matched normal volunteers. Levels of calcidiol were lower in patients with CF than in controls and lower in Massachusetts than in Arizona in both study groups. Controls in Arizona had higher (P less than 0.05) levels of calcitriol than in Massachusetts throughout the year. All control subjects in both states had higher levels of calcitriol than did patients with CF. Patients in Massachusetts had significantly lower levels of calcitriol in winter than in summer. Summer levels of calcitriol in CF were significantly higher in Massachusetts than in Arizona; during winter, lower levels were found in Massachusetts than in Arizona. Mean bone density in patients with CF was 88% and 89% of normal American standards in Massachusetts and Arizona, respectively. These data indicate a seasonal, sunlight-related influence on levels of vitamin D metabolites in patients with CF receiving approximately 1000 IU vitamin D per day. Older patients with CF with progressively diminishing sunlight exposure may be at increased risk for development of osteopenia. The detected radiographic abnormalities of bone mineralization may also be related to malabsorptive deficiencies of calcium and phosphorus.

Calcium metabolism and its control--a review

It is now established that avians can only utilize the cholecalciferol form of vitamin D, which must be converted to the hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] to perform normal calcium metabolism. Although 1,25(OH)2D3 is the final active form of vitamin D, hens fed only this form of vitamin D do not have normal hatchability of eggs. The problem appears to be caused by abnormal calcification and development of the embryonic beak. This appears to be caused by inadequate transport of 1,25(OH)2D3 into the egg. Although 1,25(OH)2D3 is not incorporated into the egg adequately, its precursor, 25-hydroxyvitamin D3 (25-OH-D3), is. The developing embryo however, can utilize 1,25(OH)2D and does so at least as early as Day 10 of incubation. During periods of maximal shell calcification and high circulating estradiol levels, the hen produces high levels of 1,25(OH)2D3. The kidney hydroxylase responsible for the final hydroxylation of the vitamin D hormone can be further stimulated by in vivo or in vitro administration of estradiol and, to a lesser extent, prolactin and parathyroid hormone. When eggs are not produced, as in the senescent or prepubertal stages of life, plasma 1,25(OH)2D3 concentrations are less than half that occurring during periods of active lay. Hens selected for their ability to produce thin or thick shells have 1,25(OH)2D3 concentrations in plasma that are positively correlated to their ability to produce egg shell.

Mineral content of the forearms of babies born to Asian and white mothers

Vitamin D deficiency is common in pregnant Asian women. The effect of maternal vitamin D deficiency on fetal skeletal mineralisation was assessed by measuring the bone mineral content of babies born to 45 Asian women, 19 Asian women who had received 1000 units of vitamin D during the last trimester, and 12 white women. The mean cord blood concentrations of 25-hydroxy vitamin D in the three groups were 5.9 +/- SE 0.9 nmol/l (2.4 +/- SE 0.4 ng/ml), 15.2 +/- 3.2 nmol/l (6.1 +/- 1.3 ng/ml), and 33.4 +/- 3.6 nmol/l (13.4 ng/ml), respectively. Despite this wide variation in values there was no significant difference in the bone mineral content (as assessed by photon absorptiometry) of the forearms of babies born to these women. This suggests that mineralization of the fetal skeleton is not impaired in maternal vitamin D deficiency. Craniotabes (skull softening) was present in seven of the 64 Asian babies. The bone mineral content in these babies was not significantly different from that of babies without this sign, and craniotabes should not therefore be taken as an indication of a generalized impairment in skeletal mineralization.

Laboratory diagnosis of inborn errors of metabolism in children

A compendium of tests available to diagnose children with inborn errors of metabolism is presented. It is intended to provide the pediatric pathologist with a rational way of using laboratory tests to investigate a patient who is suspected of having an inborn error of metabolism. It is intended to bridge the gap between the ward and the laboratory by providing a framework for investigations of such children.

Do breast and bottle fed babies require vitamin supplements?

In the present stage of knowledge, the following approach to vitamin supplementation seems reasonable. Breast-fed full-term infants should be supplemented with vitamin D (400 I.U./day). Supplementation of bottle-fed infants will depend upon the vitamin content of the formula which is being used. Pre-term and LBW infants may need higher amounts of vitamin D (800-1 000 I.U./day) and should be supplemented with vitamin E (1-2 I.U./day), C (50 mg/day), B6 (30-60 mcg/day) and folic acid (50-100 mcg/day) when they are being fed formulas with high amounts of PUFA and proteins or with iron. Multivitamin supplementation appears to be acceptable for infants of very low birth weight (less than 1 500 g) until they reach a body weight of at least 2 000 grams or a caloric intake of 300 kcal/day. Finally, the present routine of administering 0.5-1.0 mg of vitamin K at birth to all infants should be continued.

Critical vitamins for low birthweight infants

All vitamins are "critical" by definition for the growing infant. However, some of them are particularly relevant to the preterm or low birthweight (LBW) infant in whom a deficiency can occur more frequently than in a full-term newborn. In LBW infants vitamin deficiency may develop due to (1) low body stores at birth, (2) low intake, (3) limited absorption, (4) increased need or utilization, (5) presence of certain clinical conditions. The first reason concerns all lipid-soluble vitamins, and particularly vitamin E and K, which cross the placenta with some difficulty. Among hydrosoluble vitamins, cord-blood levels of vitamin B6 have been shown to be abnormally low in preterm infants. Low intake can occur because of low vitamin levels in milk or because of delayed and/or insufficient feeding. Limited intestinal absorption of vitamins in LBW infants has only been demonstrated for vitamin E and folic acid. The rapid post-natal growth may lead to increased vitamin utilization. In some clinical conditions particularly high intakes of certain vitamins are indicated. In our opinion, the really "critical" vitamins in LBW infants are vitamin D, E, K and folic acid, for which routine supplementation can be recommended, and possibly vitamins C and B6 under special circumstances.

Hypoparathyroidism and pseudohypoparathyroidism in childhood

Although relatively uncommon, the conditions of hypoparathyroidism and pseudohypoparathyroidism in childhood provide an exciting diagnostic and therapeutic challenge. Knowledge of calcium-phosphate homeostasis has progressed rapidly over the past few years so that our understanding of the basic pathophysiological mechanisms has increased tremendously. However, further clinical and basic scientific research will, no doubt, unravel further variations of the various disease entities described.

Isolation and identification of 23,25-dihydroxyvitamin D3, an in vivo metabolite of vitamin D3

Vitamin D supplemented rats produce a metabolite of 25-hydroxy[3 alpha-3H]vitamin D3 that is easily separated from known metabolites by using high-performance liquid chromatography. The production of this metabolite in vivo as well as 1,25-dihydroxyvitamin D3, 24(R),25-dihydroxyvitamin D3, and 25-hydroxyvitamin D3 26,23-lactone is largely if not totally eliminated by nephrectomy. Kidney homogenates from vitamin D supplemented chickens incubated with 25-hydroxyvitamin D3 produce significant quantities of the new, unknown metabolite. This metabolite was isolated in pure form from such incubation mixtures by using both straight-phase and reversed-phase high-performance liquid chromatography. This metabolite has been positively identified as 23,25-dihydroxyvitamin D3 by ultraviolet absorption spectrophotometry, mass spectrometry, and derivatization. This structure was confirmed by chemical synthesis of both C-23 stereoisomers. Although the natural product exactly comigrates with one of the synthetic isomers, the exact stereochemistry of the natural product remains unknown. It is possible that this new metabolite is an intermediate in the biosynthesis of 25-hydroxyvitamin D3 26,23-lactone.

The biochemical basis of renal osteodystrophy and post-menopausal osteoporosis: a view from the vitamin D system

The vitamin D endocrine system is now known to play an essential role in the regulation of plasma calcium and phosphorus concentrations and in overall organismal calcium economy. These two basic functions of the vitamin D endocrine system have provided important new insight into several disease states. The two disease states discussed here are the genesis of renal osteodystrophy and of post-menopausal osteoporosis. It seems likely that defects of alternations in the vitamin D system play important roles in the development of these disease states. Successful treatment undoubtedly will involve the vitamin D system and most likely the active form of vitamin D, 1,25- (OH)2D3.

Decreased 1,25-dihydroxycholecalciferol and increased 25-hydroxy- and 24,25-dihydroxycholecalciferol in tissues of rats treated with thyroxine

The effect of thyroxine on the metabolism of vitamin D was investigated in rats. Vitamin D depleted rats were repleted by injections of radiolabelled cholecalciferol or 25-hydroxycholecalciferol (25OHD3). After 3 weeks, a state of hyperthyroidism was induced by daily injections of L-thyroxine for 21 days. The lipid extracts of the Plasma and tissues were analyzed by successive Sephadex LH-20 and high pressure liquid chromatography. The concentrations of 25OHD3 and of 24,25-dihydroxycholecalciferol (24,25 (OH)2D3) were significantly higher and those of cholecalciferol and of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) were significantly lower in the plasma and tissues of animals treated with thyroxine than in controls. The present study suggests that alterations in the metabolism of vitamin D may be involved in the disturbances of calcium metabolism observed in hyperthyroidism.