Formation and Identification of 3-Keto-5-aminohexanoic Acid, a Probable Intermediate in Lysine Fermentation

A neutral, heat-labile compound (II) has been detected as a product of lysine fermentation by cell-free extracts of Clostridium SB4. A 2 to 5% yield of II from lysine is observed when coenzyme A is omitted from an otherwise complete reaction mixture. 3,5-Diaminohexanoate appears to be the immediate precursor of Compound II since it can be converted to II under conditions not permitting the utilization of lysine or β-lysine. Under favorable conditions, 3,5-diaminohexanoate is enzymically converted to Compound II in 20 to 30% yield. The cofactors required for the reaction are NAD and α-ketoglutarate or pyruvate; the latter is active only when lactic dehydrogenase is also added. The formation of Compound II appears to occur by a reversible NAD-dependent oxidative deamination of 3,5-diaminohexanoate. The keto acids function as oxidants in the reoxidation of NADH by the appropriate dehydrogenases. Although Compound II has not been isolated as a pure compound because of its instability, chemical and enzymic studies indicate that its structure is 3-keto-5-aminohexanoate. Compound II is reduced by borohydride to a heat-stable hydroxyamino acid, and is decarboxylated to a basic methyl ketone. The latter reacts with hypoiodite to give iodoform and β-aminobutyrate; therefore, it appears to be 2-keto-4-aminopentane.