Biotin holocarboxylase synthetase deficiency

Figure 12-2. Metabolic pathways involving the four biotin-dependent carboxylases. The solid rectangular blocks indicate the locations of the enzymes ACC, acetyl-CoA carboxylase PMCC, P-methylcrotonyl-CoA carboxylase PC, pyruvate carboxylase PCC, propionyl-CoA carboxylase. Isolated deficiencies of the first three carboxylases (mitochondrial) have been established (isolated ACC deficiency has not been confirmed). At least the activities of the three mitochondrial carboxylases can be secondarily deficient in the untreated multiple carboxylase deficiencies, biotin holocarboxylase synthetase deficiency and biotinidase deficiency. Lowercase characters indicate metabolites that are frequently found at elevated concentrations in urine of children with both multiple carboxylase deficiencies. The isolated deficiencies have elevations of those metabolites directly related to their respective enzyme deficiency.

Both multiple carboxylase deficiencies are characterized by deficient activities of the three mitochondrial carboxylases in peripheral blood leukocytes prior to biotin treatment. The carboxylase activities increase to near normal or normal after treatment with pharmacological doses of biotin. Patients with biotin holocarboxylase synthetase deficiency have deficient activities of the three mitochondrial carboxylases in fibroblasts incubated in medium with low biotin concentrations (containing only the biotin contributed by fetal calf serum added to the medium for cell growth), whereas patients with biotinidase deficiency have normal carboxylase activities under these conditions. The activities of the carboxylases in biotin holocarboxylase synthetase deficiency become near normal to normal when cultured in medium supplemented with high concentrations of biotin. 

Biotinidase deficiency and biotin holocarboxylase synthetase deficiency can be definitively diagnosed by direct enzymatic assay. Biotinidase activity in plasma or serum is usually determined by using the artificial substrate, biotinyLp-aminobenz< >ate. If biotinidase activity is present, then biotin is cleaved, releasing jD-aminobenzoatc. The / -aminobenzoate then is reacted with reagents that result in the development of purple color that can be quantitated colorimetrically. In the absence of biotinidase activity,/ -aminobenzoate is not liberated. Biotinidase activity in patients with an isolated carboxylase deficiency or biotin holocarboxylase synthetase deficiency is normal. 

Weiner DL, Grier RE, Wolf (1985) A bioassay for determining biotinidase activity and for discriminating biocytin from biotin using holocarboxylase synthetase-deficient cultured fibroblasts. J Inherit Metab Dis 8 101-102... 

To determine the type of multiple carboxylase deficiency, blood was obtained to determine the biotin holocarboxylase synthetase activity in leukocytes, and serum was sent to determine the biotinidase activity. The results of the serum biotinidase activity returned first and indicated less than 1% of mean normal serum activity, confirming that the child had profound biotinidase deficiency (less than 10% of mean normal serum biotinidase activity). Subsequently, biotin holocarboxylase synthetase activity was found to be normal. Although many states screen for biotinidase deficiency in the newborn period, this child was bom in a state where newborn screening for biotinidase deficiency is not performed. 

Individuals with untreated biotinidase deficiency develop biotin deficiency because they cannot recycle endogenous biotin. The biotin deficiency subsequently results in the lack of substrate for biotin holocarboxylase synthetase. Without the availability of biotin to be added to the apocarboxylases, multiple carboxylase deficiency occurs, and the abnormal metabolites accumulate. 

MetabolicaUy, biotin is of central importance in lipogenesis, gluconeogen-esis, and the catabolism of branched-chain (and other) amino acids. There are two well-characterized biotin-responsive inborn errors of metabolism, which are fatal if untreated holocarboxylase synthetase deficiency and biotinidase deficiency. In addition, biotin induces a number of enzymes, including glu-cokinase and other key enzymes of glycolysis. Biotinylation of histones may be important in regulation of the cell cycle. 

Holocarboxylase synthetase deficiency can be diagnosed prenatally by assessing the response of carboxylase activity in cultured amniocytes (obtained by amniocentesis) to the addition of biotin, or by the detection of methylcitric and hydroxyisovaleric acids in the amniotic fluid. Prenatal therapy, by giving the mother 10 mg of biotin per day, results in sufficiently elevated fetal blood concentrations of biotin to prevent the development of organic acidemia at birth. 

Biotinidase Deficiency Genetic lack of biotinidase results in the late-onset variant of multiple carboxylase deficiency. Patients generally present later in life than those with holocarboxylase synthetase deficiency (Section 11.2.2.1) and have a lower than normal blood concentration of biotin. Culture of fibroblasts in media containing low concentrations of biotin results in normal activities of carboxylases, and holocarboxylase synthetase activity is normal. 

Early work on the intracellular fractionation of biotin in a variety of tissues indicated that a significant amount of biotin was associated with the nuclear fraction of the cell (7,8). The biotin content of biotin-deficient rat liver is about one tenth of that of normal rat liver, and a significant 75% of this was present in the nuclear fraction, indicating that nuclear biotin was conserved in the deficient animal (9,10). Nuclear biotin was noncovalently bound to protein (2). The recent identification of the presence of biotin holocarboxylase synthetase (HCS) in the nuclear fraction of various tissues explains the earlier observation (11). However, the fact that nuclei assayed negatively for any of the biotin carboxylases would suggest a function for biotin in the nucleus other than as the prosthetic group of biotin containing carboxylases. 

Biotin has a role in cellular processes such as survival, differentiation, and development. A role for biotin in the synthesis of specific proteins has been identified (28). Biotin holocarboxylase synthetase (HCS) catalyzes the biotinylation of apocarboxylases. HCS mRNA is significantly reduced in the biotin-deficient rat. A regulatory role for biotin in the control of biotin HCS mRNA levels via signaling cascades involving guanylate cyclase and cGMP-dependent protein kinase has been proposed (29). 

Holocarboxylase synthetase deficiency [3, 4] is the classic infantile form of multiple carboxylase deficiency. Untreated it is uniformly fatal, while early diagnosis and treatment with biotin usually lead to the disappearance of all of the manifestations of the disease. Life-threatening illness is associated with massive ketosis and metabolic acidosis. A bright red cutaneous eruption may cover the body, and there is alopecia totalis. Immune function, both T and B cell, may be defective. 

Since this first case report, some 12 patients who have shown features of 3-methylcrotonyl-CoA carboxylase deficiency have been reported in the literature. The majority (7) of these patients appear to have multiple carboxylase deficiencies probably due to holocarboxylase synthetase deficiency and are responsive to D-biotin therapy. Early diagnosis is essential in order to avoid the possible fatal consequences of the diseases. The number of cases now reported permit some degree of classification, but the degree of heterogeneity of presentation necessitates their description in some detail and the important features are given below. 

Table 11.1 Abnormal Urinary Organic Acids in Biotin Deficiency and Multiple Carboxylase Deficiency from Lack of Holocarboxylase Synthetase or Biotinidase...

Biotin acts to induce glucokinase, phosphofructokinase, and pyruvate kinase (key enzymes of glycolysis), phosphoenolpyruvate carboxykinase (a key enzyme of gluconeogenesis), and holocarboxylase synthetase, acting via a cell-surface receptor linked to formation of cGMP and increased activity of RNA polymerase. The activity of holocarboxylase synthetase (Section 11.2.2) falls in experimental biotin deficiency and increases with a parallel increase in... 

Hyperammonemia occurs in biotin deficiency and the functional deficiency associated with lack of holocarboxylase synthetase (Section 11.2.2.1) and bio-tinidase (Section 11.2.3.1). In deficient rats, the activity of ornithine carbamyl-transferase is two - thirds of that in control animals, as a result of decreased gene expression, although the activities of other urea cycle enzymes are unaffected (Maeda etal., 1996). 

Most dietary biotin is bound to protein, the amide linkage being broken prior to absorption. At least eight children have been described who have multiple carboxylase deficiency with low activities of several of the biotin-requiring carboxylases, i.e., multiple carboxylase deficiency (Table 38-1). Pharmacological doses of biotin restored the activities of the carboxylases in these patients, indicating that the defect was not in the apocarboxylases. Thus, the defect is presumably in the intestinal transport system, in holocarboxylase synthetase, or in some step in cellular uptake or intracellular transport of biotin. 

Biotin deficiency may be caused by inborn errors on other proteins involved in biotin homeostasis biotidinase, the sodium-dependent multivitamin transporter and holocarboxylase synthetase (Zempleni et al. 2008). A congenital deficiency of either of these proteins may create impairments in essential metabolisms, causing clinical signs with various intensities. 

Burri, B. J., Sweetman, L. and Nyhan, W. L. (1985) Heterogeneity of holocarboxylase synthetase in patients with biotin-responsive multiple carboxylase deficiency. Am. J. Hum. Genet, 37, 326. 

In the normal turnover of cellular proteins, holo-carboxylases are degraded to biocytin or biotin linked to an oligopeptide containing at most a few amino acid residues (Figure 1). Biotinidase releases biotin for recycling. Genetic deficiencies of holocarboxylase synthetase and biotinidase cause the two types of multiple carboxylase deficiency that were previously designated the neonatal and juvenile forms. 

Disorders in the metabolism of 3-methylcrotonyl-CoA are due to deficient activity in vivo of the 3-methylcrotonyl-CoA carboxylase enzyme system. As stated in the introduction to this section, this is a biotin-dependent enzyme system in common with other mitochondrial carboxylase enzymes, in which the D-biotin is attached to the carboxylase apoenzyme to form the active holocarboxylase by holocarboxylase synthetase. The biotin-dependent enzyme systems have been extensively reviewed elsewhere (Moss and Lane, 1971 Wood and Barden, 1977 Lynen, 1979), but some comment on the mechanism of enzyme activation and action is warranted here. Most of the available information on these aspects has been obtained from work with micro-organisms, and the mammalian enzyme systems have been relatively little studied. However, the mechanism and metabolic pathways involved appear to be similar for both micro-organisms and animals. The reaction is dependent on ATP and magnesium ions and initially depends on the coupling... 

It is apparent that several abnormalities in this system may lead to deficient metabolism of 3-methylcrotonyl-CoA and hence an associated abnormal organic aciduria. Defects of the apocarboxylase affecting either active site could produce an isolated 3-methylcrotonyl-CoA carboxylase deficiency. Deficient activity of holocarboxylase synthetase would lead to multiple carboxylase deficiency and a similar disorder would be expected if a defect occurred in biotin uptake by the cell or transport into the mitochondria. It would be surprising, however, if the patients with the latter disorders would be responsive in vivo to biotin therapy and the molecular basis for the response in other cases and hence of the exact nature of the underlying primary defects remains to be elucidated by further study. 

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