The role of catabolism in controlling tissue concentrations of nicotinamide nucleotide coenzymes.

A previous report from this laboratory (Bender, D.A., Magboul, B.I. and Wynick, D. (1982) Brit. J. Nutr. 48, 119-127) suggested that the hydrolysis of the nicotinamide nucleotides NAD and NADP may be an important factor in controlling the tissue content of these coenzymes. Further studies presented here support this suggestion. Both nuclear poly(ADPribose) synthetase and microsomal NAD glycohydrolase showed activity towards both NAD+ and NADP+, and the two nucleotides were mutually competitive. The reduced nucleotides, NADH and NADPH, were not substrates for either enzyme. In rats that were maintained for 24 h under conditions of hypoxia (O2/N2, 1:9) there was an increase in the proportion of nicotinamide nucleotides present in the liver in the reduced form, and an increase in the total concentration of nucleotides in the liver. In rats that were maintained for 24 h under conditions of hyperoxia (O2/N2, 7:3) there was no change in either the proportion of nicotinamide nucleotides in the liver present in the reduced form or in the total tissue control of the nucleotides. There was an increase in the urinary excretion of kynurenine suggesting an increase in the oxidative metabolism of tryptophan.

The preferred route of kynurenine metabolism in the rat.

It has been suggested (Ueda, T., Otsuka, H. and Goda, K. (1978) J. Biochem. 84, 687-696) that direct cleavage of kynurenine, catalysed by kynureninase, followed by microsomal hydroxylation of the resultant anthranilic acid, may provide an alternative to the established pathway of kynurenine metabolism that involves direct hydroxylation followed by cleavage to 3-hydroxyanthranilic acid. To test this suggestion, anthranilic acid was administered to rats; there was no increase in either the concentration of nicotinamide nucleotides in the liver or the urinary excretion of N1-methyl nicotinamide. However, injection of either kynurenine or 3-hydroxyanthranilic acid did increase the concentration of nicotinamide nucleotides in the liver. The kinetics of kynurenine hydroxylase (Km = 1.8 +/- 0.6.10(-5) mol/l) and kynureninase (Km = 2.5 +/- 0.8.10(-4) mol/l, liver steady-state kynurenine = 4.9 +/- 0.9 mumol/kg) are such that the preferred route of kynurenine metabolism is probably by way of hydroxylation rather than cleavage.

Lethal familial pellagra-like skin lesion associated with neurologic and developmental impairment and the development of cataracts.

A new hereditary defect of tryptophan metabolism is described in a Sudanese family with a high degree of consanguinity. It has an autosomal recessive pattern of inheritance. The condition manifests as a pellagra-like skin rash within 8 weeks after birth, with signs of cerebellar ataxia and developmental retardation. Cataracts develop early, and to date none of the ten affected children has survived beyond 2 years of age. Biochemically, the condition is characterized by an apparent impairment of the ability to synthesize quinolinic acid and nicotinamide nucleotides from tryptophan, which might be due to abnormally high activity of the enzyme picolinate carboxylase.

The effects of oestrogen administration on tryptophan metabolism in rats and in menopausal women receiving hormone replacement therapy.

The effects of the administration of oestrogens on the activity of hepatic tryptophan oxygenase have been assessed both directly (by measurement of enzyme activity in vitro) and indirectly (by measurement of urinary excretion of tryptophan metabolites) in rats, and indirectly in menopausal women receiving hormone replacement therapy. Intraperitoneal administration of 500 micrograms of oestradiol or ethinyl oestradiol/kg body wt had no effect on the activity of tryptophan oxygenase in homogenates of liver from mature (13-week-old) female rats. Both adrenalectomy and ovariectomy led to a reduction in the activity of tryptophan oxygenase in homogenates of liver from mature rats; again there was no effect of giving 500 micrograms of oestradiol/kg body wt by intraperitoneal injection. Intraperitoneal administration of 210 micrograms of oestrone sulphate/kg body wt for 1 or 2 days before killing, or its incorporation in the diet for up to 8 weeks at an equivalent dose rate, had no effect on the activity of tryptophan oxygenase in homogenates of liver from ovariectomized 6-14-week-old female rats. Intraperitoneal administration of 500 micrograms oestradiol/kg body wt to intact mature female rats together with 500 mg tryptophan/kg body wt caused a reduction in the urinary excretion of xanthurenic and kynurenic acids, kynurenine and N1-methyl nicotinamide. When peri- and post-menopausal women were treated with ethinyl oestradiol (20 micrograms/day) or piperazine oestrone sulphate (3 mg/day) for 3 months, there was an increase in the concn of tryptophan in plasma, with no change in the urinary excretion of xanthurenic and kynurenic acids and kynurenine. This study provides no evidence for the induction of tryptophan oxygenase by oestrogens in rats or human beings.

B vitamins in the nervous system.

The coenzyme functions of the B vitamins in intermediatry metabolism are well established; nevertheless, for none of them is it possible to determine precisely the connection between the biochemical lesions associated with deficiency and the neurological consequences. Although there is convincing evidence of a neurospecific role for thiamin and other B vitamins, in no case has this role been adequately described. Similarly, the neurochemical sequelae of intoxication by massive amounts of vitamins (so-called mega-vitamin therapy or orthomolecular medicine) remain largely unexplained.

Vitamin B6 requirements and recommendations.

The principal metabolic function of vitamin B6 is in amino acid metabolism, although the greater part of the body's vitamin B6 is in muscle, associated with glycogen phosphorylase, and the vitamin also has an important role in the actions of steroid hormones. It is usual to calculate vitamin B6 requirements relative to protein intake. An adequate intake to meet the requirements of virtually the whole population is generally considered to be 15 micrograms/g dietary protein. This is the basis of RDAs in most countries, although differences of interpretation and application of the experimental data result in widely differing recommendations from different authorities. Current RDAs range between 1.5 and 2.2 mg/d. A minimum safe intake, below which an individual would have a high probability of deficiency, is 11 micrograms/g dietary protein. Higher intakes are required in pregnancy and lactation (although there are problems in determining the requirement of the infant), and possibly also in the elderly. Average intakes of vitamin B6 in developed countries meet the target of 15 micrograms/g dietary protein, although there is biochemical evidence of inadequate vitamin B6 nutritional status in 10-25 per cent of the population. It is not know whether this has any clinical significance; it is unlikely that normal patterns of food intake would permit a significantly greater average intake of the vitamins without fortification or the use of supplements. There is little evidence that pharmacological doses of vitamin B6 have any beneficial effect. Neurological damage has been reported at extremely high intakes (in excess of 500 mg/d), and even more modest doses (50-100 mg/d) cannot be regarded as being without hazard.

Nonpoint sources of volatile organic compounds in urban areas-relative importance of land surfaces and air.

Volatile organic compounds (VOCs) commonly detected in urban waters across the United States include gasoline-related compounds (e.g. toluene, xylene) and chlorinated compounds (e.g. chloroform, tetrachloroethane [PCE], trichloroethene [TCE]). Statistical analysis of observational data and results of modeling the partitioning of VOCs between air and water suggest that urban land surfaces are the primary nonpoint source of most VOCs. Urban air is a secondary nonpoint source, but could be an important source of the gasoline oxygenate methyl-tert butyl ether (MTBE). Surface waters in urban areas would most effectively be protected by controlling land-surface sources.

Do we really know vitamin and mineral requirements for infants and children?

Infants and young children require vitamins and minerals not only to replace losses through metabolic turnover, but also to increase body reserves as they grow. For infants up to six months of age it is generally assumed that breast milk provides adequate intakes, and average breast milk composition provides the basis of reference intakes for this age group, and the basis for infant formula. It is questionable to what extent the micronutrient content of breast milk reflects the infant's requirements as opposed to the mother's nutritional status. For obvious ethical reasons there have been very few (if any) experiments to determine the micronutrient requirements of infants and children, although studies of intakes in areas where deficiency is common provide an estimate of minimum requirements in some cases. Different national and international authorities have approached the problem of estimating reference intakes for infants and young children in four main ways: linear interpolation between adequate intakes estimated from breast milk composition and the experimentally-determined requirements of young adults; extrapolation forwards from breast milk composition and backwards from the requirements of young adults on the basis of metabolic body weight (body weight0.75), corrected by growth factors; extrapolation backwards from the requirements of young adults on the basis of the energy density of the diet; and by factorial calculation. Despite this, there is considerable concordance between the figures published by different authorities as a result of rounding off to avoid spurious precision.

Optimum nutrition: thiamin, biotin and pantothenate.

The metabolism of glucose is deranged in thiamin deficiency, but once any deficiency has been corrected there is no further effect of increased thiamin intake on the ability to metabolize glucose through either pyruvate dehydrogenase (EC 1.2.4.1) and the citric acid cycle, or the pentose phosphate pathway, in which transketolase (EC 2.2.1.1) is the thiamin-dependent step. It has been suggested that the Wernicke-Korsakoff syndrome is associated with a genetic variant of transketolase which requires a higher than normal concentration of thiamin diphosphate for activity. This finding would suggest that there may be a group of the population who have a higher than average requirement for thiamin, but the evidence is not convincing. There are no estimates of biotin requirements, but either coenzyme saturation of erythrocyte pyruvate carboxylase, or the excretion of 3-hydroxy-isovalerate (perhaps after a test dose of leucine) could be used to assess requirements in depletion-repletion studies. Biotin deficiency leads to impaired glucose tolerance, but it is unlikely that glucose tolerance could be used to assess optimum biotin status, since other more common factors affect glucose tolerance to a greater extent. Plasma triacylglycerol and nonesterified fatty acids are moderately elevated in pantothenic acid deficiency. However, this is unlikely to be useful in assessing pantothenate status, since again, other more common factors affect plasma lipids. To date there are no biochemical indices of adequate pantothenate nutrition, and no estimates of requirements.

Inhibition in vitro of the enzymes of the oxidative pathway of tryptophan metabolism and of nicotinamide nucleotide synthesis by benserazide, carbidopa and isoniazid.

The effects of three hydrazine derivatives on the enzymes of the tryptophan oxidative pathway and of nicotinamide nucleotide synthesis have been studied using preparations from rat liver. The compounds used were Benserazide and Carbidopa, two inhibitors of aromatic amino acid decarboxylase used together with dopa in the treatment of Parkinson's disease, and the anti-tubercular agent isoniazid. All three drugs inhibited tryptophan oxygenase and kynureninase, at concentrations that are likely to be encountered in vivo following administration to patients or experimental animals. Isoniazid, but not Benserazide or Carbidopa, also inhibited 3-hydroxy-anthranilate oxidase and nicotinamide phosphoribosyltransferase. However, these two enzymes were of the drug far in excess of those likely to be encountered in vivo. On the basis of the in vitro enzyme inhibition studies, it is not possible to explain why patients treated with isoniazid (without supplementary vitamin B(6)) develop clinical pellagra, while those treated with Benserazide or Carbidopa do not, despite biochemical evidence of niacin deficiency. It is suggested that this difference may be due either to differences in the intake of dietary niacin in these two groups of patients, or more probably to differences in the metabolism of the drugs and in their interactions with enzymes in vivo that are not apparent in vitro.