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Biomedical subjects

B E Nordin

Publications and source records attributed to B E Nordin.

At least 19 recordsLinked to original sources
PubMed1999

Nucleotide determinants for tRNA-dependent amino acid discrimination by a class I tRNA synthetase.

The high accuracy of the genetic code relies on the ability of tRNA synthetases to discriminate rigorously between closely similar amino acids. While the enzymes can detect differences between closely similar amino acids at an accuracy of about 1 part in 100-200, a finer discrimination requires the presence of the cognate tRNA. The role of the tRNA is to direct the misactivated amino acid to a distinct catalytic site for editing where hydrolysis occurs. Previous work showed that three nucleotides at the corner of the L-shaped tRNA were collectively required. Here we show that each of these nucleotides individually contributes to the efficiency of editing. However, all are dispensable for the chemical step of hydrolysis. Instead, these nucleotides are required for translocation of a misactivated amino acid from the active site to the center for editing.

Acylation↗
PubMed1999

RNA determinants for translational editing. Mischarging a minihelix substrate by a tRNA synthetase.

The fidelity of protein synthesis requires efficient discrimination of amino acid substrates by aminoacyl-tRNA synthetases. Accurate discrimination of the structurally similar amino acids, valine and isoleucine, by isoleucyl-tRNA synthetase (IleRS) results, in part, from a hydrolytic editing reaction, which prevents misactivated valine from being stably joined to tRNAIle. The editing reaction is dependent on the presence of tRNAIle, which contains discrete D-loop nucleotides that are necessary to promote editing of misactivated valine. RNA minihelices comprised of just the acceptor-TPsiC helix of tRNAIle are substrates for specific aminoacylation by IleRS. These substrates lack the aforementioned D-loop nucleotides. Because minihelices contain determinants for aminoacylation, we thought that they might also play a role in editing that has not previously been recognized. Here we show that, in contrast to tRNAIle, minihelixIle is unable to trigger the hydrolysis of misactivated valine and, in fact, is mischarged with valine. In addition, mutations in minihelixIle that enhance or suppress charging with isoleucine do the same with valine. Thus, minihelixIle contains signals for charging (by IleRS) that are independent of the amino acid and, by itself, minihelixIle provides no determinants for editing. An RNA hairpin that mimics the D-stem/loop of tRNAIle is also unable to induce the hydrolysis of misactivated valine, both by itself and in combination with minihelixIle. Thus, the native tertiary fold of tRNAIle is required to promote efficient editing. Considering that the minihelix is thought to be the more ancestral part of the tRNA structure, these results are consistent with the idea that, during the development of the genetic code, RNA determinants for editing were added after the establishment of an aminoacylation system.

Base Sequence↗
PubMed1999

Biochemical variables in pre- and postmenopausal women: reconciling the calcium and estrogen hypotheses.

There is controversy as to whether the rise in urinary calcium at the menopause is the cause or the result of the rise in bone resorption at that time. In an attempt to resolve this issue, we have compared the relevant biochemical variables in 102 premenopausal volunteers (mean age 37 years; range 21-52) and 86 apparently normal postmenopausal women (mean age 55 years; range 40-60). We measured the fasting serum calcium, creatinine, proteins, electrolytes and intact parathyroid hormone (PTH), and the urinary calcium and creatinine both after an overnight fast and in a 24-h collection. We calculated serum calcium fractions, creatinine clearance and the notional tubular maximum reabsorptive capacity for calcium. Creatinine excretion and clearance were lower in the post- than in the premenopausal women after correction for surface area and age. Total serum calcium was higher in the post- than in the premenopausal women but this was accounted for by the higher ligand concentrations in the former. Fasting and 24-h urinary calcium were also higher in the post- than in the premenopausal women due in part to the former's higher filtered load of calcium (due to their higher serum complexed calcium) but mainly to their reduced tubular reabsorption of calcium despite their slightly raised serum PTH. Our analysis resolves the rise in urinary calcium at the menopause into its two components: increased filtered load and reduced tubular reabsorption. The changes in these two variables, neither of which can be attributed to increased bone resorption, produce an increase in calcium requirement that is sufficient to account for postmenopausal bone loss. However, the translation of this menopausal increase in calcium requirement into an increase in bone resorption at near-normal serum PTH levels requires some menopause-dependent change in the responsiveness of the bone to calcium demand. We suggest that this change may occur at the level of the osteoclasts and that estrogen may modify the calcium feedback setpoint in these cells in a manner analogous to calcitonin. This model resolves the apparent conflict between the estrogen and calcium hypotheses and explains the synergism between these two treatment modalities.

Adult↗
PubMed1999

The effects of low dose norethisterone on biochemical variables in postmenopausal women.

Norethisterone 2.5 mg/day was administered to 26 postmenopausal women (aged 54-79 years) with varying degrees of osteoporosis and with a forearm bone mineral density value more than 2 SD below the young normal mean. Fasting blood and urine samples were collected and radiocalcium absorption measured at baseline and after treatment for a median period of 4 months. There were significant falls in serum calcium and its fractions, phosphate, alkaline phosphatase and cholesterol (HDL and LDL), and significant rises in serum chloride and parathyroid hormone. In the urine, there were significant falls in calcium, sodium and hydroxyproline. These changes were in close agreement with our previously reported responses to norethisterone 5 mg/day. We conclude that norethisterone in a dose of 2.5 mg/day is probably as effective as 5 mg/day in reducing bone resorption in postmenopausal women with low bone density.

Aged↗
PubMed1999

Bone growth from 11 to 17 years: relationship to growth, gender and changes with pubertal status including timing of menarche.

The tempo and change in bone growth during puberty in relation to physical growth is described in a cohort of 56 boys and 52 girls. Distal forearm bone width, mineral content and volumetric density, anthropometry and pubertal status were measured at ages 11, 13, 15 and 17 y, and bone age at 17 y. Bone width and mineral content increased independently with age for each pubertal stage. Volumetric density fell during early puberty and then increased rapidly. Maximal increase of all bone variables occurred earlier in girls than in boys and earliest for bone width, then mineral content, then density. In girls most change occurred in the 12 mo before and after menarche. The degree of tracking was similar to that for height. Bone growth followed physical growth but at a slower tempo. By age 17 y boys had attained 86% of the reference adult bone mineral content and volumetric density; girls had attained 93% of the reference adult bone mineral content and 94% of volumetric density. Those skeletally mature at 17 y had greater mineral content and volumetric density. To maximize peak bone mass, modifiable environmental factors should be optimized before the onset of puberty and be maintained throughout this period of rapid growth and beyond attainment of sexual maturity.

Adolescent↗
PubMed1999

Familial and environmental influences on bone growth from 11-17 years.

The influences on bone growth of familial factors, nutrition and physical activity are described in a cohort of 108 children (56M, 52F). Distal forearm bone width, mineral content and volumetric density, anthropometry, pubertal status, nutritional intake and physical activity were measured at ages 11, 13, 15 and 17 y. Parental forearm bone status was also determined. Both mothers' and fathers' bone variables were significant predictors of the respective children's bone variables, but heritability estimates were greater between mothers and their children than between fathers and their children. By age 17 y boys had attained 101%, 85% and 89% of their fathers' height, bone mineral content and volumetric density, respectively; girls had attained 103%, 95% and 98% of their mothers' height, bone mineral content and volumetric density, respectively. There were no consistent associations among nutrient variables and bone status or rate of change in bone status. However, there was a significantly greater increase in bone mineral content and density from 11-17 y in those girls with consistently high calcium intake. There were no significant correlations between physical activity and bone values or rate of change of bone values. Age, gender, pubertal status, height, weight and parental bone values accounted for 80%, 71% and 49% of the variance of bone mineral content, bone width and volumetric density, respectively and 52%, 55% and 58% respectively of the variance of change in these variables. After age, gender, sexual maturity and body size, heritability accounts for the greatest variance in bone values through adolescence.

Adolescent↗
PubMed1998

Nutrition, osteoporosis, and aging.

Loss of bone is an almost universal accompaniment of aging that proceeds at an average rate of 0.5-1% per annum from midlife onwards. There are at least four nutrients involved in this process: calcium, salt, protein, and vitamin D, at least in women. The pathogenesis of osteoporosis in men is more obscure. Calcium is a positive risk factor because calcium requirement rises at the menopause due to an increase in obligatory calcium loss and a small reduction in calcium absorption that persist to the end of life. A metaanalysis of 20 calcium trials shows that this process can generally be arrested by calcium supplementation, although there is some doubt about its effectiveness in the first few years after menopause. Salt is a negative risk factor because it increases obligatory calcium loss; every 100 mmol of sodium takes 1 mmol of calcium out of the body. Restricting salt intake lowers the rate of bone resorption in postmenopausal women. Protein is another negative risk factor; increasing animal protein intake from 40 to 80 g daily increases urine calcium by about 1 mmol/day. Low protein intakes in third world countries may partially protect against osteoporosis. Vitamin D (sometimes called a nutrient and sometimes a hormone) is important because age-related vitamin D deficiency leads to malabsorption of calcium, accelerated bone loss, and increased risk of hip fracture. Vitamin D supplementation has been shown to retard bone loss and reduce hip fracture incidence in elderly women.

Aged↗
PubMed1998

Activation of microhelix charging by localized helix destabilization.

We report that aminoacylation of minimal RNA helical substrates is enhanced by mismatched or unpaired nucleotides at the first position in the helix. Previously, we demonstrated that the class I methionyl-tRNA synthetase aminoacylates RNA microhelices based on the acceptor stem of initiator and elongator tRNAs with greatly reduced efficiency relative to full-length tRNA substrates. The cocrystal structure of the class I glutaminyl-tRNA synthetase with tRNAGln revealed an uncoupling of the first (1.72) base pair of tRNAGln, and tRNAMet was proposed by others to have a similar base-pair uncoupling when bound to methionyl-tRNA synthetase. Because the anticodon is important for efficient charging of methionine tRNA, we thought that 1.72 distortion is probably effected by the synthetase-anticodon interaction. Small RNA substrates (minihelices, microhelices, and duplexes) are devoid of the anticodon triplet and may, therefore, be inefficiently aminoacylated because of a lack of anticodon-triggered acceptor stem distortion. To test this hypothesis, we constructed microhelices that vary in their ability to form a 1.72 base pair. The results of kinetic assays show that microhelix aminoacylation is activated by destabilization of this terminal base pair. The largest effect is seen when one of the two nucleotides of the pair is completely deleted. Activation of aminoacylation is also seen with the analogous deletion in a minihelix substrate for the closely related isoleucine enzyme. Thus, for at least the methionine and isoleucine systems, a built-in helix destabilization compensates in part for the lack of presumptive anticodon-induced acceptor stem distortion.

Amino Acyl-tRNA Synthetases↗
PubMed1998

Calcium supplementation suppresses bone resorption in early postmenopausal women.

In order to establish whether calcium supplementation suppresses bone resorption in early postmenopausal women and whether any response is related to calcium absorption status, we studied 22 healthy women (median age 52 years) all within 5 years of the menopause. Urine was collected between 9.00 p.m. and 9.00 a.m., and 9.00 a.m. and 9.00 p.m., (2 days) and a fasting blood and spot urine sample was obtained at 9 a.m. On the first day, 5 microCi of 45Ca in 250 ml water with 20 mg calcium carrier as the chloride was given at 9.00 a.m. and a further blood sample was obtained at 10.00 a.m. to measure calcium absorption. A 1 g calcium load was given at 9.00 p.m., immediately before the second 24-hour urine collection. There was a rise in plasma ionized calcium (1.18 +/- 0.010 mmol/liter versus 1. 21 +/- 0.011 mmol/liter, P < 0.01) and a fall in plasma PTH (4.2 +/- 0.34 pmol/liter versus 3.5 +/- 0.31 pmol/liter, P < 0.01) from baseline after the calcium load, and a trend for the magnitude of the change in PTH to be inversely related to calcium absorption (r = -0.33, P = 0.13). In the fasting spot urine samples, there were falls in hydroxyproline (OHPr/Cr; 14.6 +/- 0.71 versus 12.6 +/- 0.83, P < 0.001), pyridinoline (Pyr/Cr; 75 +/- 2.8 versus 70 +/- 3.5, P < 0.05), and deoxypyridinoline (Dpd/Cr; 22.7 +/- 1.2 versus 19.5 +/- 1. 1, P < 0.005) after the calcium load. The calcium load suppressed urinary Dpd/Cr between 9.00 p.m. and 9.00 a.m. (P < 0.005), but not between 9.00 a.m. and 9.00 p.m. We conclude that acute administration of a 1 g calcium load suppresses bone resorption in early postmenopausal women, probably by decreasing PTH secretion.

Amino Acids↗
PubMed1998

Intestinal calcium absorption in men with spinal osteoporosis.

OBJECTIVE: To investigate the role of serum 1,25-dihydroxyvitamin D (1,25D) in the decreased calcium absorption found in men with osteoporosis. DESIGN: Prospective study of patients referred to a university teaching hospital clinic for investigation of possible osteoporosis. PATIENTS: Male patients referred for investigation for osteoporosis, from 1981 to 1995, because of specific risk factors or radiological suspicion of osteoporosis. Men with vertebral compression fractures were compared with those without. MEASUREMENTS: Height and weight, radiocalcium absorption, serum 1,25D and fasting urinary calcium and hydroxyproline excretion. RESULTS: The men with vertebral fractures had higher fasting urinary hydroxyproline excretion (P = 0.003) and lower calcium absorption (P = 0.002) than the men without. Calcium absorption was positively related to 1,25D in both groups but the estimated calcium absorption at zero 1,25D was lower in the osteoporotic than the normal group. 1,25D was lower in the osteoporotic group than in the normal group. However this difference could only explain about half of the difference in calcium absorption between the groups. CONCLUSIONS: Calcium absorption is low in men with osteoporosis. About half of the deficit is due to low serum 1,25-dihydroxy vitamin D levels but there appears, in addition, to be some intestinal resistance to its effect on calcium absorption.

Adult↗
PubMed1998

Modification and validation of a single-isotope radiocalcium absorption test.

UNLABELLED: This study was done to establish and allow for the influence of body weight on plasma radioactivity after administering radiocalcium to measure calcium absorption. METHODS: We administered 5 microCi 45Ca in 20 mg of calcium carrier in 250 ml distilled water to 103 premenopausal volunteers over the age of 40 yr, after an overnight fast. Venous blood was withdrawn when the dose was given (to serve as a blank) and exactly 60 min later, and the counts were determined in a liquid scintillation counter. After the exclusion of three outliers, the fraction of the administered dose per liter of plasma at 60 min was a curvilinear inverse function of body weight and a positive linear function of the reciprocal of body weight, with an r value of 0.45 (p < 0.001). This latter relationship then was used to correct the plasma radioactivity to a standard body weight of 65 kg, in which the volume of distribution of the dose was assumed to be 10 liters. This yielded the estimated fraction of the dose circulating at 1 hr, which then was converted into a fractional absorption rate from our previously published equation. RESULTS: In the 100 volunteers, the mean value of the radiocalcium absorption rate (termed alpha2, to distinguish it from our original calculation) was 0.75/hr, with 98 of the 100 values falling between 0.30 and 1.20. The value alpha2 was significantly related to serum calcitriol in these 100 volunteers (r = 0.29; p = 0.003) and in 89 normal postmenopausal women (r = 0.46; p < 0.001). It also was significantly related to the 24-hr urine calcium in the same 89 women (r = 0.48; p < 0.001) and to net calcium absorption corrected for intake in balance studies on another 103 postmenopausal women (r = 0.44; p < 0.001). In most respects, alpha2 was marginally superior to alpha1 but, unlike alpha1, was independent of body weight. CONCLUSION: The modified low-carrier radiocalcium absorption test is a valid indicator of calcium absorption status over a wide range of calcium intakes and is independent of body weight.

Body Weight↗
PubMed1997

The response to calcitriol therapy in postmenopausal osteoporotic women is a function of initial calcium absorptive status.

Calcitriol is used in the treatment of osteoporosis but the indications for its use have not been clearly defined. Because it stimulates calcium absorption, we have tended to select osteoporotic patients with low calcium absorption for this therapy and now report the results. We measured the hourly fractional rate of calcium absorption (alpha) with 45Ca and fasting urinary calcium/creatinine (Ca/Cr) and hydroxyproline/creatinine (OHPr/Cr) in 103 postmenopausal women aged 68 (0.67SE) years with vertebral compression fractures (77) or forearm or vertebral bone density below the young normal range (26). They were given 0.25 microg daily of calcitriol (Rocaltrol, Roche, Basle, Switzerland) with a 1 g calcium supplement daily for 6-12 weeks, when the biochemical tests were repeated. Initial OHPr/Cr was inversely related to initial alpha (P = 0.001) and positively to initial Ca/Cr (P < 0.001). alpha rose on therapy from 0.47 (0.018) to 0.59 (0.018) per hour (P < 0. 001) and OHPr/Cr fell in the whole group from 19.1 (0.83) to 13.8 (0. 58) (P < 0.001). The change in alpha on therapy (corrected for the "regression to the mean effect") was inversely related to initial alpha (P < 0.001) as was the change in OHPr/Cr (P = 0.001). There was no relationship, however, between initial Ca/Cr and either the rise in alpha or the fall in OHPr/Cr on therapy. The data support the concept that low calcium absorption is a cause of negative calcium balance in postmenopausal osteoporosis and that the effectiveness of calcitriol therapy is inversely related to the initial rate of calcium absorption.

Adult↗
PubMed1997

Relations between calcium intake, calcitriol, polymorphisms of the vitamin D receptor gene, and calcium absorption in premenopausal women.

The relations between calcium absorption, dietary calcium intake, 1,25-dihydroxyvitamin D3 (calcitriol), and vitamin D receptor (VDR) gene polymorphisms were evaluated in 99 healthy women who were approaching menopause (mean age: 47 y, range: 43-53 y). Dietary calcium was assessed by food-frequency questionnaire and calcium absorption was measured by a single-isotope radiocalcium test. VDR alleles were classified according to the presence (b, t, a) or absence (B, T, A) of the BsmI, TaqI, and ApaI restriction enzyme cutting sites. Radiocalcium absorption was positively related to serum calcitriol (r = 0.23, P < 0.05) and inversely related to dietary calcium intake (r = -0.26, P < 0.01). There was, however, no significant relation (r = 0.10) between serum calcitriol concentrations and dietary calcium. Radiocalcium absorption was higher in the bbaaTT haplotype (P < 0.05) and the aa genotype (P < 0.05), polymorphisms said to be associated with a higher bone density. We conclude that serum calcitriol and dietary calcium are independent determinants of calcium absorption in premenopausal women and that VDR gene polymorphisms influence calcium absorption.

Adult↗