Microbial L‐ and D‐Lactate Selective Oxidoreductases as a Very Prospective but Still Uncommon Tool in Commercial Biosensors

Lactic acid, being in natural L-form, is an almost universal metabolite of all living organisms. It is also an important biotechnological product used as a food additive, component of many industrial products, pharmaceuticals, cosmetics and, recently, as a monomer for the production of biodegradable poly-L-lactate. D-Lactate is another stereoisomer known as a by-product of some fermentation processes and formed also in the colon and blood under some pathophysiological conditions, namely, diabetes, short bowel syndrome, and others. Thus, both lactate stereoisomers are important biomarkers of some diseases, indicators of food quality, and markers of sport medicine for assessing the level of athlete training. Therefore, the development of new highly selective biosensors for the routine analysis of these biomarkers is very important. Some electrochemical biosensors for lactate analysis are commercialized and exploit bacterial enzymes: NAD+-dependent L- or D-lactate dehydrogenases or lactate oxidase. The yeast cells produce another type of very specific response toward lactate enzymes: L-lactate:cytochrome c-oxidoreductase (EC 1.1.2.3, flavocytochrome b2, FC b2) and D-lactate: ferricytochrome c-oxidoreductase (EC 1.1.2.4, DLDH), which are specific toward L- and D- stereoisomers of lactate, respectively. The main properties of both enzymes are absolute selectivity toward lactate isomers and non-specificity to the nature of electron acceptors, as well as an ability to direct electron transfer to electrodes. The listed properties make the enzymes very suitable for their application in electrochemical biosensorics. This Review is devoted mainly to the characterization of FC b2- and DLDH-based laboratory prototypes of electrochemical biosensors to promote scientific and possible commercial interest in such L-lactate- and D-lactate-specific bioanalytical tools.