Requirements biotin

Two particularly interesting aspects of the pyruvate carboxylase reaction are (a) allosteric activation of the enzyme by acyl-coenzyme A derivatives and (b) compartmentation of the reaction in the mitochondrial matrix. The carboxy-lation of biotin requires the presence (at an allosteric site) of acetyl-coenzyme A or other acylated coenzyme A derivatives. The second half of the carboxylase reaction—the attack by pyruvate to form oxaloacetate—is not affected by CoA derivatives. 

A variety of biotin-requiring microorganisms have been used to assay biotin Saccharomyces cerevisiae (H6), Lactobacillus casei (S2), Lactobacillus arabinosus (now L. plantarum ATCC No. 8014) (W14), Micrococcus sodonensis (Al), Neurospora crassa (H10), and Rhizobium tri-folii (W7). None have been applied successfully for assaying biotin in biologic fluids. Because the flagellate Ochromoms danica had a specific and sensitive biotin requirement (A2), it was utilized as a reagent for biotin in blood, serum, urine, brain, and liver tissue (B3b). 

Carbon dioxide is a normally unreactive material, and combination with biotin requires the input of energy (from ATP). Carbon dioxide is usually present... 

Biotin required for growth and normal function by animals, yeast, and many bacteria is seldom found in deficiency in humans because the intestinal bacteria synthesize it in sufficient quantity to meet requirements. Biotin deficiency does occur, however, 111 animals fed raw whiles of eggs. The egg white contains a protein, avidin, which combines with biotin, and tins complex is not broken down by enzymes of the gastrointestinal tract. Hence, a deficiency develops. 

Molasses was the most common carbon source until being replaced by glucose for reasons of easier downstream processing. Sufficient amounts (over 30 /jlg/L) of biotin must be included in the medium to prevent the excretion of glutamic acid. This biotin requirement was previously met by using molasses as the carbon source and must now be added exogenously. The fermentation runs at temperatures of about 28-33°C, and pH 6-8. High aeration is desirable. The final product concentration is around 100-140 g/L, and the fermentation time is 48-72 hr. The yield of lysine on carbohydrate is about 40-50 percent. The formation of lysine from sucrose can be represented as follows,... 

More recently, similar signs of biotin deficiency have been observed in patients receiving total parenteral nutrition for prolonged periods, after major resection of the gut. The signs resolve after the provision of biotin, but again there have been no studies of the amounts of biotin required intakes have ranged between 60 to 200 /xg per day (Mock et al., 1985). 

It is apparent from the discussion in Section 11.3 that there is Utde information concerning human biotin requirements and no evidence on which to base recommendations. Average intakes of biotin range between 15 to 70 /rg per day. Such intakes are obviously adequate to prevent deficiency, and the safe and adequate range of biotin intakes is set at 10 to 200 /xg per day (Department of Health, 1991 Scientific Committee for Food, 1993). The U.S./Canadian adequate intake for adults is 30 /xgper day (Institute of Medicine, 1998). 

In Streptomyces, it is assumed that streptavidin has an antibiotic role it is secreted together with a low molecular weight inhihitor of hiotin synthesis, stra-vidin. It has been suggested that avidin in eggs has a similar role, to protect the developing embryo from (biotin-requiring) bacteria that penetrate the shell. Alternatively, because cells in culture can take up and utilize avidin-biotin. 

Some vitamins undetfio a rather unique transformation prior to becoming functional. They are covalently attached to specific enzymes. Biotin, for example, is covalently bound to the biotin-requiring enzymes. Pantothenic acid, in a modified form, is covalently bound to fatty acid synthase. Riboflavin, following conversion to thecofaclor form, is bound to succinate dehydrogenase, as well as to a few other enzymes requiring riboflavin-based cofactors. 

Biotin deficiency in animals results in alopecia, a. scaly dermatitis, anorexia, and eventually death. Some of the biochemical changes occurring in rats consuming a biotin-deficient diet containing raw egg white are illustrated in Figure 9.33. The animals consumed the diet for up to 30 days. The activities of three biotin-requiring enzymes were determined in the livers of rats killed at the indicated times. The en iymes measured were acetyl-CoA carboxylase ( , propionyl-Co A carboxylase (O), and pyruvate carboxylase (A). 

Figure 4.46 shows the first step (acetyl-CoA carboxylase catalyzed) in the fatty acid synthesis pathway. The enzyme is biotin-requiring, and the product is malonyl-CoA. Note that the activities of about 100 different enzymes have been foxmd to be controlled by phosphorylation (Shacter et ah, 1986). In all cases, the phosphorylation is reversible. The phosphate donor may be ATP or GTP. 

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. 

Four amino acids are converted to propionyl CoA, which is car-boxylated in a biotin-requiring reaction to form methylmalonyl CoA, o which is rearranged to form succinyl CoA in a reaction that requires... 

Dietary Reference intakes. These have been difficult to determine. There has been some speculation that humans might obtain part of their biotin requirements from the intestinal flora in the colon. The question that has not been adequately answered is whether there is significant absorption of bacteria-pro-duced biotin from the colon. 

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. 

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