<p>Sodium pyruvate is an important biological molecule, which serves as a substrate for several enzymes in the main metabolic pathways of carbohydrates. This is the first report showing that, along with its well-known antioxidant properties, sodium pyruvate also can act as a photosensitizer when exposed to long-wavelength UV radiation (λ = 325 nm) corresponding to its low intensity n → π^* absorption band. The sensitizing properties of pyruvate were studied in relation to the lactate dehydrogenase (LDH) enzyme, for which pyruvate serves as a substrate. The pyruvate-sensitized photodecomposition of tryptophan and cystine amino acid residues has been shown to lead to photoinactivation of the enzyme. The photochemical process is inhibited by adding free radical quenchers and antioxidant enzymes to the irradiated LDH-pyruvate mixture as well as by replacing H₂O with D₂O. The rate of this process is also enhanced by reducing the concentration of dissolved oxygen. A decisive role was proposed for radical processes (type I photochemical reactions) in the pyruvate-sensitized photoinactivation of LDH.</p>