Plasma phenylalanine and tyrosine levels revisited in heterozygotes for hyperphenylalaninaemia.

We examined the value of the fasting plasma phenylalanine/tyrosine ratio obtained in an ordinary clinical setting for assessing the probability of being a heterozygote for hyperphenylalaninaemia. This biochemical test was found to be of little value in those with a high (66%) prior risk of heterozygosity, because it could not reduce the risk below 12%. However, in a population with a prior risk of only 2%, it discriminates the 3% with a 19% risk from the 97% with a risk of 1.5% or less. This simple method could usefully be applied to such a population, in order to select those at higher risk for further investigation using molecular genetics.

Study of the relationship between estimates of enzyme kinetic parameters.

Previous indications of an intrinsic relationship between estimates of Km and Vmax calculated from the Michaelis-Menten equation have been explored further. A mathematically linear relationship could be established for the estimates of the two parameters. The relationship--the trend line--holds whether or not the experimental error is linked to the rate of reaction, the substrate concentration or both, provided that the distribution of errors is symmetrical. The practical implication is that enzyme variants with low values of Km and Vmax may not be distinguishable from those with high values of Km and Vmax.

Interrelation between the metabolism of L-isoleucine and L-allo-isoleucine in patients with maple syrup urine disease.

The nonprotein amino acid L-allo-isoleucine is formed endogenously in maple syrup urine disease patients from (R)-3-methyl-2-oxo-pentanoic acid. During strict metabolic balance, the plasma L-allo-isoleucine/L-isoleucine ratio correlates inversely with the residual activity of the branched-chain 2-oxoacid dehydrogenase in fibroblasts and thus constitutes a relevant in vivo parameter of the severity of the metabolic defect in MSUD patients.

Diarrhoea due to breast milk: case of fucose intolerance?

An unusual form of diarrhoea is reported that was relieved when breast feeding was stopped. Chromatography to estimate sugars in the faeces should be performed for all infants with unexplained diarrhoea before changes are made in the diet.

Effect of temperature and sample preparation on performance of ion-moderated partition chromatography of organic acids in biological fluids.

A thorough investigation of the behavior of organic acids on the Bio-Rad Aminex cation exchange resin was prompted by both the limitations of, and a number of inexplicable inconsistencies found in, previously published papers using an identical system. In order to stabilise the elution order of various acids it was necessary to analyse samples at a higher temperature than previously recommended. This temperature (50 degrees C) decreased the retention times of all acids permitting the analysis of both aromatic and aliphatic acids within the same 45-min run. Preparation of an acidic fraction of biological fluids improved specificity, allowed direct comparison of urine and plasma profiles and by control of the conditions interference by urate could be substantially reduced. Retention data are given for more than 90 acidic metabolites including nearly 40 of clinical significance and a number derived from diet and drug therapy.

Effect of partial ornithine carbamoyltransferase deficiency on urea synthesis and related biochemical events.

The biochemical response to an intravenous alanine load of 0.25 g/kg was studied in nine adult female relatives of children with ornithine carbamoyltransferase deficiency. Six were classified as affected by partial deficiency and three as unaffected. The plasma ammonium concentration showed no change after the alanine load in the unaffected group, but marked increases occurred in all but one of the affected groups. The maximum rate of urea synthesis after the alanine load was decreased by 37% (P = 0.02) and delayed by 43% (P = 0.02) in the affected group. In the affected group a low rate of urea synthesis was associated with high urinary orotate excretion, high maximum plasma ammonium concentration and delay in the time taken to reach the maximum rate of urea synthesis (Kendall concordance W = 0.55, P less than 0.05). The effects of a higher dose of alanine and of oral protein were compared. The alanine load of 0.25 g of alanine/kg body weight was shown to provide an adequate stimulus to urea synthesis with a more rapid return of ammonium concentration to the pre-load level than with the protein load. The implication of these results in determining the distribution of flux control of urea synthesis, the discrepancy between them and predicted results and the necessary modifications to quantitative simulations are discussed.

Acetoacetyl CoA thiolase deficiency presenting as ketotic hypoglycemia.

We report two children who presented with hypoglycemia and metabolic acidosis in whom acetoacetyl-CoA thiolase (EC 2.3.1.9) measured in fibroblast homogenates was deficient. Deficiency of this enzyme is normally associated with urinary excretion of 2-methylacetoacetate and in one child the urinary excretion of 2-methylacetoacetate, 2-methyl-3-hydroxybutyrate, and tiglylglycine was raised. By contrast, in the other child, the urinary excretion of these metabolites was very low even during ketoacidosis and following an isoleucine load. We suggest that this could be due to deficiency of the extrahepatic isoenzyme, a defect that may be responsible for some of the cases of "ketotic hypoglycemia."

Biotin-reversible neurodegenerative disease in infancy.

Two siblings with consanguineous parents began having myoclonic jerks at age 5 months after introduction of mixed feeding. There was later developmental regression. The elder girl died without diagnosis aged 1 year, after prolonged continuous hyperventilation. The younger sibling did not have metabolic acidosis when first investigated for myoclonus and hypotonia aged 5 months. At 9.5 months, when intermittently decerebrate and hyperventilating, she had a metabolic acidosis with elevated blood lactic, pyruvic and beta-hydroxybutyric acids, and beta-hydroxyisovaleric aciduria. On the assumption that she had beta-methylcrotonyl-CoA carboxylase deficiency she was started on biotin, 10 mg daily. Within 36 h there was dramatic clinical and biochemical improvement. Previously defective eye movement control and gaze became normal, hyperventilation ceased, and excessive organic acid excretion in urine was abolished. She remains on long-term biotin and at age 2 years her development appears normal in all respects. Fibroblast culture however revealed normal quantities of the enzymes beta-methylcrotonyl-CoA carboxylase, propionyl-CoA carboxylase and pyruvate carboxylase. Irrespective of niceties of enzyme and organic acid biochemistry, the clinician must be aware of biotin-reversible regressive brain disease which may present before manifest metabolic acidosis.

Validation of a method for measuring the short-term rate of urea synthesis after an amino acid load.

The response of the plasma concentration of urea to the oral and intravenous administration of alanine was studied in healthy adult humans. The instantaneous rate of urea synthesis was calculated by using a model-dependent procedure. The errors in this procedure were calculated and it was shown that analytical precision and sampling frequency, and the estimates of the distribution volume and elimination fluxes, were adequate to determine the synthesis parameters. A direct test of the compartmental model was made by the intravenous injection of exogenous urea. The one-compartment model with first-order elimination gave a good fit to the experimental results at times greater than 8 min after the injection. Both oral and intravenous loads of alanine had dose-dependent effects on the rate of urea synthesis. There was no evidence of a limit to the maximum possible rate of urea synthesis in these experiments and the values obtained were similar to published results for different stimuli and methods of measurement. The rate of synthesis increased more rapidly after intravenous loads and subjective side-effects were less severe. The intravenous administration of alanine appears to be a suitable stimulus for urea synthesis.

Determination of the rate of urea synthesis from serial measurements of plasma urea concentration after an alanine load: theoretical and methodological aspects.

A method is described by which the rate of synthesis of urea can be calculated from the change of plasma concentration of urea after an alanine load. The results can be expressed in terms of f, the maximum increase in the rate of urea synthesis, and t, the time at which urea synthesis reaches its maximum. These parameters are calculated by an algebraic curve-fitting technique which is suitable for a desk computer. The method removes the need for isotopic analysis and urine collections. The effect of various errors and experimental conditions on the calculated synthesis parameters is investigated.

Hyperphenylalaninaemia of various types among three-quarters of a million neonates tested in a screening programme.

A total of 795 382 infants born in north London was screened for phenylketonuria using the Guthrie test between October 1969 and December 1978. During this period it became recognised that phenylketonuria is not a single disease entity but one that encompasses a number of disorders of differing clinical and biochemical severity. The overall incidence of persistent hyperphenylalaninaemia was of the order of 7 per 100 000 births (or 1 in 15 000) and all the early treated patients made normal developmental progress. During the study there was an appreciable fall in the incidence of uncomplicated transient hyperphenylalaninaemia with or without tyrosinaemia. This reduction coincided with the change in infant feeding practice in the UK which led to lower intakes of protein and phenylalanine. It was concluded that any infant found to have a persistent blood phenylalanine concentration of 240 mumol/1 (4 mg/100 ml) or greater should be followed closely.

Inherited disorders of 3-methylcrotonyl CoA carboxylation.

The clinical course of 4 patients who had reduced activities of 3-methylcrotonyl CoA carboxylase (also called 3-methylcrotonylglycinuria) is described. Two children presented with a metabolic acidosis, one in the neonatal period and the other with episodes of acidosis that started in the second year of life. In the other 2 children neurological symptoms were prominent, one having infantile spasms and the other developmental regression with a skin rash and alopecia. Three of the children responded well to oral biotin and dietary protein restriction but the fourth, despite a biochemical response to biotin, has a severe neurological handicap. The clinical presentation of inborn errors of 3-methylcrotonyl CoA carboxylase is variable. Metabolic acidosis may not be conspicuous and instead neurological features may predominate.

Changing incidence of neonatal hypermethioninaemia: implications for the detection of homocystinuria.

The Guthrie test was used to measure blood methionine concentrations in 670 764 neonates during the period from May 1970 to December 1977. Raised values (greater than 4 mg/100 ml; 268 mumol/l) were found in 147 babies (6--14 days old) and 55 of these still had raised values when retested 2--6 weeks later. 48 infants had transient hypermethioninaemia of at least 3 weeks' duration, one had a more persistent form associated with abnormal liver function tests, 3 had different forms of homocystinuria, and one infant, who was asymptomatic at the time of detection, had hypermethioninaemia associated with a rapidly fatal form of tyrosinamiea (tyrosinosis). Two infants could not be followed up. Transient hypermethioninaemia has not been detected in this laboratory since 1975. There was a greatly reduced incidence of transient hypermethioninaemia in girls after 1972 and in boys after 1975; this may have been due to recent changes in infant practices in the UK. Homocystinuria was last detected in this laboratory in 1972; the apparent change in incidence is significant (P less than 0.05) and suggests that the diagnostic value of this screening procedure should be reassessed.