Catabolism of Biotin

Dietary deficiency of biotin sufficient to cause clinical signs is extremely rare in human beings, although it may be a problem in intensively reared poultry. However, there is increasing evidence that suboptimal biotin status may be relatively common, despite the fact that the vitamin is widely distributed in many foods, is synthesized by intestinal flora, and there is an efficient mechanism for conserving biotin after the catabolism of biotin-containing enzymes. 

As a result of this resorption and the protein binding of plasma biotin, which reduces filtration at the glomerulus, renal clearance of biotin is only 40% of that of creatinine. This efficient conservation of biotin, together with the recycling of biocytin released from the catabolism of biotin-containing enzymes, may be as important as intestinal bacterial synthesis of the vitamin in explaining the rarity of deficiency. 

Biocytin is hydrolyzed by biotinidase, which acts on free or peptide-incorporated biocytin to release biotin, but has no general peptidase or esterase activity. Biotinidase is most active toward free biocytin, but it will also release biotin from biocytin-containing peptides. The activity decreases as the size of the peptide increases, so it is likely that in vivo the catabolism of biotin-containing enzymes is by proteolysis, followed by biotinidase action, rather than the release of biotin, leaving the apoenzyme as a substrate for proteolysis. Biotinidase is found in all tissues, including the pancreatic juice and intestinal mucosa. 

Biotin deficiency in experimental animals is teratogenic, and a number of the resultant birth defects resemble human birth defects. Up to half of pregnant women have elevated excretion of 3-hydroxy-isovaleric acid (Section 11.4), which responds to supplements of biotin, in the first trimester, suggesting that marginal stams may be common in early pregnancy and may be a factor in the etiology of some birth defects. This may be the result of increased catabolism of biotin as a result of steroid induction of biotin catabolic enzymes there is increased excretion of bisnorbiotin and biotin sulfoxide (Zempleni and Mock, 2000a Mock et al., 2002). 

Figure 43.1 Apocarboxylase biotinylation by holocarboxylase synthetase. Holocarboxylases are formed by biotinylation of apocarboxylases and free biotin is subsequently released by catabolism of biotin-containing enzymes.

S Iwahara, DB McCormick, LD Wright. Isolation and characterization of bisnorbio-tin, dehydrobisnorbiotin, and tetranorbiotin from catabolism of biotin by Pseudomonas sp. Methods Enzymol 18A 404-409, 1970. 

Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

Biotin (6.24) consists of an imidazole ring fused to a tetrahydrothiophene ring with a valeric acid side chain. Biotin acts as a co-enzyme for carboxylases involved in the synthesis and catabolism of fatty acids and for branched-chain amino acids and gluconeogenesis. 

In the ruminant mammary tissue, it appears that acetate and /3-hydroxybutyrate contribute almost equally as primers for fatty acid synthesis (Palmquist et al. 1969 Smith and McCarthy 1969 Luick and Kameoka 1966). In nonruminant mammary tissue there is a preference for butyryl-CoA over acetyl-CoA as a primer. This preference increases with the length of the fatty acid being synthesized (Lin and Kumar 1972 Smith and Abraham 1971). The primary source of carbons for elongation is malonyl-CoA synthesized from acetate. The acetate is derived from blood acetate or from catabolism of glucose and is activated to acetyl-CoA by the action of acetyl-CoA synthetase and then converted to malonyl-CoA via the action of acetyl-CoA carboxylase (Moore and Christie, 1978). Acetyl-CoA carboxylase requires biotin to function. While this pathway is the primary source of carbons for synthesis of fatty acids, there also appears to be a nonbiotin pathway for synthesis of fatty acids C4, C6, and C8 in ruminant mammary-tissue (Kumar et al. 1965 McCarthy and Smith 1972). This nonmalonyl pathway for short chain fatty acid synthesis may be a reversal of the /3-oxidation pathway (Lin and Kumar 1972). 

In the degradation of isoleucine, (3 oxidation proceeds to completion in the normal way with generation of acetyl-CoA and propionyl-CoA. However, in the catabolism of leucine after the initial dehydrogenation in the (3-oxidation sequence, carbon dioxide is added using a biotin enzyme (Chapter 14). The double bond conjugated with the carbonyl of the thioester makes this carboxylation analogous to a standard (3-carboxylation reaction. Why add the extra C02 ... 

Vitamin B12 is essential for the methylmalonyl-CoAmutase reaction. Methylmalonyl-CoA mutase is required during the degradation of odd-chain fatty acids and of branched-chain amino acids. Odd-chained fatty acids lead to propionyl-CoA as the last step of P-oxida-tion. Methylmalonyl-CoA can be derived from propionyl-CoA by a carboxylase reaction similar to that of fatty acid biosynthesis. The cofactor for this carboxylation reaction is biotin, just as for acetyl-CoA carboxylase. The reaction of methylmalonyl-CoA mutase uses a free radical intermediate to insert the methyl group into the dicar-boxylic acid chain. The product is succinyl-CoA, a Krebs cycle intermediate. The catabolisms of branched-chain lipids and of the branched-chain amino acids also require the methylmalonyl-CoA mutase, because these pathways also generate propionyl-CoA. 

Biotin, an essential water-soluble B-complex vitamin, is the coenzyme for four human carboxylases (Fig. 12-2) These include the three mitochondrial enzymes pyruvate carboxylase, which converts pyruvate to oxaloacetate and is the initial step of gluconeogenesis propionyl-CoA carboxylase, which catabolizes several branched-chain amino acids and odd-chain fatty acids and 3-methylcrotonyl-CoA carboxylase, which is involved in the catabolism of leucine and the principally cytosolic enzyme, acetyl-CoA carboxylase, which is responsible for the... 

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. 

The products of the isoleucine catabolic pathway are propionyl-CoA and ace-tyl-CoA valine catabolism produces one molecule of propionyl-CoA and two molecules of carbon dioxide. Propionyl-CoA is further cataboli25ed to succinyl-CoA, an intermediate of the Krebs cycle (Figure 8.7). This pathway is also used for catabolism of the short-chain fatty acid propionic acid, after its conversion to the thiol ester form by thiokinase. The first step in propionyl-CoA breakdown is catalyzed by propionyl-CoA carboxylase, a biotin-requiring enzyme. The second step is catalyzed by methylmalonyl-CoA mutase, a vitamin Bi2-requiring enzyme. 

There is reason to conclude that vitamin deficiency might contribute to arteriosclerosis. There is a correlation between elevated homocysteine levels and incidence of cardiovascular disease (59). There is debate as to whether homocysteine contributesto the dam e of cells on the interior of blood vessel or whether homocysteine is a marker of intensive cell repair and formation of replacement cells. Nevertheless, administration of pyridoxine, folic acid, and (yanocobalamin are being recommended along with the two antioxidant vitamins, a-tocopherol and ascorbic acid for arteriosclerosis. Vitamin Bg is required for two of the steps in the catabolism of homocysteine to succinyl CoA (Fig. 8.52). Note in Fig. 8.52 (bottom) that biotin and a coenzyme form of cobalamin also are required for... 

In leucine catabolism (Scheme 62c), the first steps leading to isovaleryl-CoA 233, X = CoA, are similar to those in the catabolism of valine 179 and isoleucine 212. When samples of (2R)- and (2S)-[2- H Jisovaleric acid 233, X = OH, were fed to biotin-deficient rats, / -hydroxyisovalerate 235 was isolated and shown to have lost the 2-pro-R hydrogen (212), thus indicating that the dehydrogenation step 233 234 had occurred with loss of this hydrogen. The hydration step 234 235 proved to be nonstereospecific for... 

After dehydrogenation to 234, X = SCoA, the catabolism of leucine 205 (Scheme 62c) differs from that of the other branched-chain amino acids. A biotin-dependent carboxylation leads to the acid 236, X = SCoA, which is hydrated to HMG-CoA 237, a compound involved in isoprenoid biosynthesis. Feeding stereospecifically labeled samples of leucine in studies of terpenoid biosynthesis indicated that the ( )-methyl group was carboxylated without isomerization of the double bond (181, 182). Messner, Cornforth et al. (215) investigated the hydration 236 = 237 catalyzed by the enzyme 3-methyl-glutaconyl-CoA hydratase (EC 4. 2. 1. 18) and showed that the reversible reaction had syn stereospecificity. 

McCormick and coworkers identified two pathways of biotin catabolism using microbial and rat models (McCormick and Wright 1971 Lee et al. 1972) (Figure 10.1). These pathways are conserved in humans (Zempleni et al. 2009), with degradation of the heterocyclic ring being the notable exception (Lee et al. 1972) (Table 10.1). The valeric acid side chain of biotin is catabolized by... 

The anticonvulsants primidone and carbamazepine inhibit biotin uptake into brush-border membrane vesicles from human intestine (Zempleni et al. 2009). Long-term therapy with anticonvulsants increases both biotin catabolism and urinary excretion of 3-hydroxyisovaleric acid. These eifects might be due to displacement of biotin from biotinidase by anticonvulsants, thereby aifecting plasma transport, renal handling or cellular uptake of biotin. 

Biotin enters tissues by a saturable transport system, and is then incorporated into biotin-dependent enzymes as the 8-aminolysine peptide, biocytin. Unlike other B vitamins, whose concentrative uptake into tissues can be achieved by facilitated diffusion followed by metabolic trapping, the incorporation of biotin into enzymes is relatively slow and cannot be considered part of the uptake process. On catabolism of the enzymes, biocytin is hydrolysed by biotinidase, permitting reutilization. 

It will be the purpose of this chapter to provide an overview of our knowledge on the catabolism of three S-containing heterocyclic cofactors, viz. lipoate, biotin, and the 8a-(S-L-cysteinyl)riboflavin, with emphasis on the fate of the molecular sulfur others will deal with the more-biosynthetic aspects of lipoic acid and biotin. 

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