Cloning of the human MCCA and MCCB genes and mutations therein reveal the molecular cause of 3-methylcrotonyl-CoA: carboxylase deficiency

3-Methylcrotonyl-CoA: carboxylase (EC 6.4.1.4; MCC) deficiency is an inborn error of the leucine degradation pathway (MIM *210200) characterized by increased urinary excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine. The clinical phenotypes are highly variable ranging from asymptomatic to profound metabolic acidosis and death in infancy. Sequence similarity with Glycine max and Arabidopsis thaliana genes encoding the two subunits of MCC permitted us to clone the cDNAs encoding the alpha- and beta-subunits of human MCC. The 2580 bp MCCA cDNA encodes the 725 amino acid biotin-containing alpha-subunit. The MCCA gene is located on chromosome 3q26-q28 and consists of 19 exons. The 2304 bp MCCB cDNA encodes the non-biotin-containing beta-subunit of 563 amino acids. The MCCB gene is located on chromosome 5q13 and consists of 17 exons. We have sequenced both genes in four patients with isolated biotin-unresponsive deficiency of MCC. In two of them we found mutations in the MCCA gene. Compound heterozygosity for a missense mutation (S535F) and a nonsense mutation (V694X) were identified in one patient. One heterozygous mutation (S535F) was found in another patient. The remaining two patients had mutations in the MCCB gene. One consanguineous patient was homozygous for a missense mutation (R268T). In the other we identified a missense mutation in one allele (E99Q) and allelic loss of the other. Mutations were correlated with an almost total lack of enzyme activity in fibroblasts. These data provide evidence that human MCC deficiency is caused by mutations in either the MCCA or MCCB gene.

Prenatal diagnosis of holocarboxylase synthetase deficiency by assay of the enzyme in chorionic villus material followed by prenatal treatment

Deficiency of holocarboxylase synthetase leads to multiple carboxylase deficiency, which is fatal in the absence of prompt diagnosis and treatment with biotin. In a pregnancy at risk for deficiency of holocarboxylase synthetase, prenatal diagnosis was performed by assay of the enzyme in chorionic villus material. The Km for biotin was 220.8 nmol/l, which was 33 times the control value of 6.6 nmol/l. Biotinyl AMP synthesis was undetectable in cultured chorionic villus material. Prenatal treatment of the mother was begun with 10 mg a day of biotin and continued through pregnancy. There was no accumulation of the characteristic metabolites in the urine at birth and prior to oral treatment of the newborn. Holocarboxylase synthetase activity was undetectable in lymphocytes and in fibroblasts of the newborn. Furthermore, the activities of all three carboxylases in fibroblasts of the infant were deficient. The newborn was clinically well and maintained on biotin treatment after birth at 20 mg per day. Carboxylase activities in lymphocytes were normal or slightly lower than the normal range.

Prenatal diagnosis and treatment of holocarboxylase synthetase deficiency

Holocarboxylase synthetase is one of two enzymes known to be involved in the metabolism of biotin. It catalyses the fixation of biotin to inactive apocarboxylases yielding active carboxylases. Deficiency of this enzyme leads to multiple carboxylase deficiency which is fatal in the absence of prompt diagnosis and treatment with biotin. In a pregnancy at risk for deficiency of holocarboxylase synthetase prenatal diagnosis was performed by assay of the enzyme in amniocytes. The Km for biotin was 62.8 nM which was 12 times the control value of 5.0 nM. The Vmax was 2 per cent of the control value. This was confirmed by assay of the activity of propionyl CoA carboxylase (20-26 per cent control), 3-methylcrotonyl CoA carboxylase (14-19 per cent control) and pyruvate carboxylase (12-30 per cent control) and demonstration of biotin responsiveness in vitro. All carboxylase activities were restored to 51-58 per cent of control when amniocytes were cultured in medium containing 1 microM biotin. Diagnosis was ultimately confirmed by assay of holocarboxylase synthetase in lymphocytes from the infant after birth. The Km for biotin of the holocarboxylase synthetase of the infant was 60.3 nM while that of a parallel control was 6.9 nM. Prenatal treatment of the mother with biotin led to a concentration of biotin of 240 nM in the serum of the infant at birth that was four times the Km of the enzyme for biotin. The infant was clinically well at birth, and organic acid analysis of the blood and urine revealed no accumulation of the characteristic metabolites.

Deficiency of biotinyl-AMP synthetase activity in fibroblasts of patients with holocarboxylase synthetase deficiency

A simple, rapid assay was developed to diagnose holocarboxylase synthetase deficiency. Holocarboxylase synthetase first catalyzes the formation of biotinyl-AMP from biotin and ATP, an activity designated as biotinyl-AMP synthetase. In the second step of the reaction, biotin is transferred from biotinyl-AMP to the enzymatically inactive apocarboxylase to form an active holocarboxylase. The assay for holocarboxylase synthetase activity therefore requires a protein apocarboxylase substrate which is not readily available. In the assay for biotinyl-AMP synthetase, hydroxylamine reacts nonenzymatically with the product of the enzymatic reaction, biotinyl-AMP, to form biotinylhydroxamate. At the end of the reaction, unreacted radioactive biotin substrate, which is negatively charged at neutral pH, is bound to an anion-exchange resin and a neutral radioactive biotinylhydroxamate product in the supernatant is counted. In fibroblasts from 11 patients with proven holocarboxylase synthetase deficiency, the mean biotinyl-AMP synthetase activity at 25 nM biotin was 4% of the control mean with a range of 0.2 to 8%. This is an improved assay because it does not require preparation of an apocarboxylase substrate and is suitable for the diagnosis of patients with holocarboxylase synthetase deficiency.

High voltage electrophoresis of amino acids in urine containing ampicillin

Analysis of amino acids is complicated by treatment with ampicillin. High voltage electrophoresis, which is convenient for the qualitative assessment of metabolic diseases, yields smears of ampicillin that mask the bands of citrulline, homocitrulline, phenylalanine, cystine, and homocystine. The addition of penicillinase prior to high voltage electrophoresis eliminates ampicillin and other penicillins and reveals these key amino acids.

Transport of the biotin dietary derivative biocytin (N-biotinyl-L-lysine) in rat small intestine

BACKGROUND

Biocytin is an important end product of intraluminal digestion of dietary protein-bound biotin. Limited studies are available regarding the ability of the small intestine to transport biocytin and about the mechanism involved. The aim of the present study was to delineate these issues.

METHODS

Transport of [3H]-biocytin was examined using everted sacs from rat intestine.

RESULTS

Mucosal-to-serosal transport of low (0.022 mumol/L) and high (5 mumol/L) concentrations of biocytin were linear for up to 20 minutes of incubation. Transport of biocytin as a function of concentration (0.022-5 mumol/L) was linear (r = 0.99) and occurred at a rate of 22,062 fmol.g tissue (wet wt)-1.15 min-1. Addition of high concentrations of unlabeled biocytin, biotin, biotin methyl ester, and lysine did not cause a significant inhibition of the transport of [3H]-biocytin. Furthermore, transport of biocytin was independent of Na+ concentration, pH, energy, and temperature. Compared with transport of equimolar concentrations of free biotin, transport of biocytin (0.022 mumol/L) was significantly lower in both the jejunum and the ileum.

CONCLUSIONS

(1) Biocytin transport in rat intestine is lower than that of free biotin and occurs via simple physical diffusion. (2) In the rat, efficient absorption and optimal bioavailability of dietary protein-bound biotin necessitates its conversion to free biotin.

A new quantitative assay for glycosaminoglycans

A direct dye-binding technique has been developed to measure total glycosaminoglycans in urine. Fifty or 100 microliters of urine was mixed with a solution of Azure A (10 mg/l) and Azure B (10 mg/l) and the resulting decrease in absorbance at 610 nm was measured. The standard curve with chondroitin sulfate C was linear from 1 to 20 micrograms/assay. The normal value for total urinary glycosaminoglycans in adults was 1.8 +/- 0.6 g/mol creatinine. Other normal values were age-dependent. The assay is inexpensive, simple, precise, sensitive and suitable for screening for the mucopolysaccharidoses in pediatric patients.

A new immunochemical assay for biotin

A double antibody technique has been developed to separate free biotin from bound biotin after competitive binding of [3H]biotin and unlabelled biotin to avidin. Antiavidin goat antibody was added followed by the addition of antigoat IgG antibody linked to agarose. Centrifugation separated the free biotin from the biotin bound to the avidin complex. The method was suitable for the detection of the amounts of biotin contained in 100-200 microliters of plasma or 5-10 microliters of urine. Normal values for the concentration of biotin in plasma and urine determined by this assay were 1.27 +/- 0.67 nmol/l and 49.1 +/- 35.7 mumol/mol creatinine, respectively.

Deficiency of carnitine palmitoyltransferase I

Defective activity of carnitine palmitoyltransferase I was demonstrated in fibroblasts derived from a patient with hypoketotic hypoglycemia. The level of activity observed was approximately 10% of the control mean. Oxidation of palmitate by intact fibroblasts was reduced to 5% of control values. The patient presented at age 14 months with seizures and was found to have marked hypoglycemia and no ketones in the urine. In response to fasting, she developed hypoglycemia, but the curves for acetoacetate and 3-hydroxybutyrate were flat. Administration of medium-chain triglycerides relieved the hypoglycemia and generated a brisk ketogenesis.

Multiple carboxylase deficiency due to deficiency of biotinidase

A patient with biotinidase deficiency was studied in whom the first admission to hospital for acidosis occurred at 5 years of age. Sensorineural abnormalities of the optic and auditory nerves antedated diagnosis and treatment with biotin, and these sensory losses did not resolve with treatment. The other clinical manifestations of the disease were highly responsive to biotin. Biotinidase was assayed using 14C-labeled natural substrate. The activity in the patient approximated 1% of the control level.

Studies on a monoclonal antibody to human factor viii coagulant activity, with a description of a facile two-site factor VIII coagulant antigen assay

IgG was purified from the ascites tumor fluid obtained from mice injected with a monoclonal cell line secreting antibody that inhibited VIII:C. With a modified Bethesda assay method (18 hr, 4 degrees C), the titer of the purified IgG was 14,000 U/mg. In a fluid-phase IRMA for VIII:CAg utilizing the Fab fragment prepared from the monoclonal IgG, two high-titer human anti-VIII:C inhibitors (IgG fractions) showed no demonstrable competition for the monoclonal VIII:CAg binding site. Conversely, neither human antibody (125I-Fab fragment) was displaced from its VIII:CAg binding site by the monoclonal IgG molecule. When the monoclonal antibody was used in a fluid-phase IRMA, slightly decreased VIII:CAg levels were found in serum. A facile one-step, two-site IRMA using Sepharose-bound human anti-VIII:C and labeled monoclonal IgG was designed. With this assay, in contrast to the finding with the fluid-phase IRMA, both the rate and apparent level of binding of VIII:CAg "sandwiched" between the two antibodies were increased approximately twofold in serum compared to the native plasma. A similar increase in rate and apparent level of binding was also found after thrombin treatment of VIII:C/vWf relative to the untreated control preparation.

Identification of platelet receptors for bovine von Willebrand factor on human platelets by a new platelet receptor test

A new test for the quantitation of platelet receptor activity for bovine von Willebrand factor in human platelet extracts is described. The test is based on the competition for the von Willebrand factor occurring between platelets, an antibody against von Willebrand factor, and platelet extracts containing the von Willebrand receptor. Bovine von Willebrand factor aggregates human platelets directly. In the presence of EDTA, anti-bovine von Willebrand factor antibody disaggregates platelet aggregates induced by bovine von Willebrand factor. Disaggregation is measured by following the decrease in light transmission accompanying disaggregation. Platelet extracts inhibited the disaggregating activity of the antibody, and the percent inhibition was directly proportional to the amount of the platelet extract added. The decrease in platelet disaggregating activity of the antibody was used as an indicator of platelet receptor activity in a system consisting of platelet-rich plasma, bovine von Willebrand factor, antibody and platelet extract. With this method we found von Willebrand factor receptors in the whole platelet extract, the Triton X-100 platelet membrane extract and in platelet extract material which bound to a bovine Willebrand factor-Sepharose 6MB affinity column.

Prostaglandin metabolism by human testis

Human testis preparations appear to carry out both the synthesis and turnover of prostaglandins. Prostaglandins and prostaglandin metabolites were extracted from whole tissue. Testis microsomes converted both endogenous and exogenous substrate to prostaglandins. Microsomal prostaglandin biosynthesis was inhibited by indomethacin. Prostaglandin E2-9-ketoreductase was present in both the cytosolic and microsomal fractions of human testis. Prostaglandin metabolism by human testis has not previously been reported.

Effect of renal microsomes and renal lysosomes on in vitro hepatic fatty acid synthesis

An inhibitor and stimulator of in vitro hepatic fatty acid synthesis are present in renal microsomes. In addition, a stimulator of fatty acid synthesis is present in renal lysosomes. Renal microsomal inhibition of hepatic fatty acid synthesis is not due to the depletion of cofactors in the system. This inhibitor appears to be located exclusively in the kidney medullary microsomes. It is destroyed by Pronase and heat treatment suggesting it may be a protein. Its effects on fatty acid synthesis may be attributed in part to ATPase activity as well as a direct effect on the hepatic fatty acid synthesizing system. A stimulator of hepatic fatty acid synthesis is present in the buffer insoluble fraction of an acetone powder preparation of renal microsomes. This stimulator is relatively heat labile and does not appear to be a phospholipid. The lysosomal stimulator of hepatic fatty acid synthesis is associated with the contents of renal lysosomes and not with the lysosomal membranes. It acts at the acetyl-CoA carboxylase step and its activity is not affected by fasting or aminonucleoside induced nephrosis.