The reaction product of enzymic dehydrogenation of alpha-hydroxy-gamma-carboxymuconic epsilon-semialdehyde (HCMS) was isolated. The analytical data (elemental analyses, IR spectra, mass spectra, proton NMR spectra, and UV spectra) showed that the product was not alpha-hydroxy-gamma-carboxymuconic acid (HCMA), the expected primary product of HCMS dehydrogenation, but a lactone of HCMA. The structure of the lactone was tentatively determined as alpha-pyrone-4,6-dicarboxylic acid. HCMS was converted stoichiometrically to the lactone by the purified NAD(P)-linked HCMS dehydrogenase. The lactone was actively metabolized by a cell-free extract prepared from Pseudomonas ochraceae grown on phthalic acid, suggesting that it may be a metabolic intermediate in the bacterial metabolism of protocatechuic acid. Furthermore, a method for the chemical synthesis of the lactose of HCMA is presented and some of its chemical and biochemical properties are described.