Combined carboxylase deficiency,

The carboxylation reactions of the carboxylase enzyme are also inhibited by avidin, specific biotin-binding protein from egg white. Experimental feeding of avidin or raw egg white results in biotin deficiency with features of combined carboxylase deficiency, since the avidin-biotin complex is not absorbed from the gut. Avidin has four biotin-binding sites and also inhibits biotin-containing enzymes by effectively decarboxylating and preventing re-carboxylation of the biotin moiety (Moss and Lane, 1971). 

A high Ala level in combination with a high Cit suggests pyruvate carboxylase deficiency (Robinson et al., 1984). 

In some patients with combined deficiency of vitamin K-dependent coagulation factors, the deficiency can be partially corrected by high doses of vitamin K, suggesting that the defect is in the affinity of the carboxylase for its coenzyme (Mutucumarana et al., 2000). 

Hydroxy- 9-methylglutaryl CoA further yields acetyl CoA and acetoacetic acid, as was shown earlier by Coon et cU. (I48). In biotin deficiency the carboxylation reaction does not occur. It was shown by Lynen et al. that the actual carboxylation is preceded by the enzymic dehydration (rf jS-hydroxyisovaleryl CoA to /8-methylcrotonyl CoA, which is the true substrate for the entry of CO2. TTiis occurs at the expense of the hydrolysis of the terminal P04 of ATP. The unsaturated intermediate is then saturated by the addition of H2O to yield the final product. The critical step of this carboxylation is the conversion of CO2 to a reactive form. The analogy of the biochemical activation of other substances through an acyl adenylate type of compound did not fit CO2 activation. The final mechanism of the activation of CO2 was derived from the discovery that the carboxylase enzyme was a biotin-protein. This observation explains earlier work 149) which indicated that biotin is a cofactor of the fatty acid-synthesizing enzyme system. When the purified carboxylase was incubated with P and ATP an exchange reaction of phosphate occurred, which was inhibited by avidin, a protein which specifically binds biotin. This indicated that the primary reaction in CO2 fixation is the combination of ATP with the biotin-protein enzyme to yield ADP biotin-protein -f P. The active CO2 is then the product of an exchange reaction between ADP and C02 which is finally attached to the biotin-protein complex. 

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