3-Hydroxyisovaleric Acid (3-HIA) is formed from the metabolism of the branched-chain amino acid leucine. Methylcrotonyl-CoA carboxylase catalyzes an essential step in this pathway and is biotin dependent. Reduced activity of this enzyme leads to an alternate pathway of metabolism resulting in 3-hydroxyisovaleric acid. 3-Hydroxyisovaleric Acid (3-HIA) is formed from the metabolism of the branched-chain amino acid leucine. Methylcrotonyl-CoA carboxylase catalyzes an essential step in this pathway and is biotin dependent. Reduced activity of this enzyme leads to an alternate pathway of metabolism resulting in 3-hydroxyisovaleric acid.
The urinary excretion of 3-HIA has been shown to be an early and sensitive indicator for marginal biotin deficiency. Elevated levels of 3-HIA in pregnant women reflect reduced or marginal biotin status. Smoking and anticonvulsant medication can also increase this metabolite as a reflection of accelerated biotin metabolism and therefore marginal deficiency.
Biotin deficiencies, certain lifestyle habits (smoking), or specific genetic conditions can reduce methylcrotonyl-CoA carboxylase activity. This reduction can lead to a buildup of 3-methylcrotonyl-CoA, which is converted into 3-hydroxyisovaleryl-CoA by the enzyme enoyl-CoA hydratase. Increased concentrations of 3-methylcrotonyl-CoA and 3-hydroxyisovaleryl-CoA can lead to a disruption of the esterified CoA:free CoA ratio, and ultimately to mitochondrial toxicity. Detoxification of these metabolic end products occur via the transfer of the 3-hydroxyisovaleryl moiety to carnitine forming 3-hydroxyisovaleric acid-carnitine or 3HIA-carnitine, which is then transferred across the inner mitochondrial membrane where 3-hydroxyisovaleric acid is released as the free acid.
3-Hydroxyisovaleric acid has been found to be elevated in smokers and in subjects undergoing long-term anticonvulsant therapy with carbamazepine and/or phenytoin. These levels are elevated due to impairment of renal reclamation of biotin. Levels may also be increased from prolonged consumption of raw egg-whites. When present in sufficiently high levels, 3-hydroxyisovaleric acid can act as an acidogen and a metabotoxin. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels.
Chronically high levels of 3-hydroxyisovaleric acid are associated with at least a dozen inborn errors of metabolism, including 3-hydroxy-3-methylglutaryl-CoA lyase deficiency, 3-methylglutaconic aciduria type I, biotinidase deficiency and isovaleric aciduria, dihydrolipoamide dehydrogenase deficiency, 3-methylcrotonyl-CoA carboxylase 1 deficiency, 3-hydroxy-3-methylglutaryl-CoA lyase deficiency, late-onset multiple carboxylase deficiency, holocarboxylase synthetase deficiency, and 3-methylcrotonyl-CoA carboxylase 2 deficiency.
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