Valine alpha-hydroxyisocaproate.

Leucine is transaminated to alpha-ketoisocaproate, which can be reduced to alpha-hydroxyisocaproate or oxidatively decarboxylated onward. The hydroxy acid therefore sits directly downstream of leucine in branched-chain amino acid handling. Supplying one raises the pool the other comes from. This is settled intermediary metabolism, not a claim about an effect.

All three branched-chain amino acids are transaminated by the same enzyme and then handled by the same branched-chain ketoacid dehydrogenase complex, so their intermediates compete for shared capacity. Valine gives alpha-ketoisovalerate and then alpha-hydroxyisovalerate, while leucine gives the isocaproate series. Naming a valine compound with an isocaproate suffix mixes the two chains. The shared pathway is real, the shared identity is not.

The branched-chain aminotransferase and the ketoacid dehydrogenase complex serve all three amino acids, so a large load of one shifts flux through the shared steps. This is why lopsided single-amino-acid dosing changes the plasma ratios of the others. The competition is at the enzyme, not at absorption alone. Direction is established, magnitude depends on the dose given.

The E1 component of the complex uses thiamine pyrophosphate to decarboxylate branched-chain keto acids. Without adequate thiamine the keto acids accumulate rather than proceeding to oxidation. This is the classic textbook link and the reason thiamine is given in maple syrup urine disease variants. Correcting a shortfall is different from adding thiamine on top of adequate status.

Riboflavin supplies FAD, which the E3 subunit uses to reoxidise lipoamide so the complex can turn over again. The same E3 protein serves pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, so riboflavin status touches all three. This is established cofactor biochemistry with no citation required. It describes capacity, not an effect of supplementation.

Transamination of leucine, isoleucine and valine to their keto acids requires PLP at the aminotransferase. Without it the amino acids cannot enter the catabolic route at all. This applies to essentially every transamination in the body, so it is not specific to branched-chain handling. State it as cofactor dependence rather than as a benefit.

Oxidative decarboxylation of branched-chain keto acids consumes NAD+ as the terminal electron acceptor within the complex. Cellular NAD+ availability therefore sets part of the ceiling on flux. Whether supplemental NAD precursors change that flux in people is not established. The stoichiometry is textbook, the supplement effect is not.

Alpha-ketoisocaproate can be oxidised by KIC dioxygenase to HMB or reduced to the alpha-hydroxy acid, so the two metabolites share an immediate parent. Products marketing leucine metabolites often carry both. Their downstream literatures are separate and should not be pooled. The shared origin is metabolic fact, the shared effect is not demonstrated.

Roughly a quarter of whey protein by weight is branched-chain amino acids, with leucine the largest share. Any metabolite of leucine is therefore already being produced from a whey-containing diet. That context matters when judging what an isolated metabolite dose adds. The composition figure is standard analytical data, not a claim.

After the dehydrogenase step, leucine catabolism passes through a biotin-dependent carboxylation before reaching acetoacetate and acetyl-CoA. Biotin shortfall shows up as a rise in 3-hydroxyisovaleric acid on organic acid testing. This is established clinical biochemistry used diagnostically. It describes pathway dependency, nothing about supplement benefit.

Acylcarnitine profiling picks up isovaleryl and isobutyryl species precisely because carnitine buffers these acyl-CoA intermediates. That buffering keeps free CoA available for continued flux. Whether supplemental carnitine changes branched-chain handling in healthy people is not established. The transport chemistry is well described.

Regulation of the complex runs through a kinase and a phosphatase, and the kinase reaction needs magnesium-bound ATP. That makes magnesium part of how flux through the pathway is turned up or down rather than part of the catalytic step itself. This is regulation, not activation. No supplement claim follows from it.