Stereospecificity of some invertebrate lactic dehydrogenases
Master of Arts
It has been assumed that only among unicellular organisms did nicotinamide adenine dinucleotide (NAD)-linked lactic dehydrogenases occur. All higher animals were thought to possess Lr-specific lactic dehydrogenases. It was discovered recently, however, that many species of invertebrates possess lactic dehydrogenases specific for the D(-) isomer of lactic acid. A study of lactic dehydrogenase stereospecificity in organisms from ten invertebrate phyla has been carried out in order to supplement the available data on the phylogenetic distribution of D-specific lactic dehydrogenases. D-specific lactic dehydrogenases were found in annelids, acanthocephala, molluscs, and arthropods. All of the platyhelminths and echinoderms tested had enzymes specific for L(+) lactic acid. In the annelids the oligochaetes and the closely related hirudinea possess If-specific lactic dehydrogenases while the polychaetes which utilized lactic acid utilized the D(-) isomer. Among the four classes of molluscs which have been studied all possessed D-specific lactic dehydrogenases. In the arthropods the chelicerates tested were specific for L-lactic acid while the mandibulates were specific for D-lactic acid. These results indicate that the occurrence of D-lactic dehydrogenases apparently falls along previously established taxonomic lines. 3. A brief review of the terminal pathways of anaerobic glycolysis in the phyla studied has also been included. Many invertebrates do not produce lactic acid as the primary end product of anaerobic carbohydrate catabolism as in vertebrate tissues. Rather they produce low levels of lactic acid and varying quantities of succinic acid and fatty acids. The pathway usually proposed for succinic acid production in these organisms involves the degradation of glucose via the Embden-Meyerhof-Parnas pathway to the level of phosphoenolpyruvate or pyruvate and then partial reversal of the tricarboxylic acid cycle. While succinic acid production is widespread among invertebrates under conditions of anoxia or low oxygen tension, its occurrence cannot be directly correlated with the distribution of D-specific lactic dehydrogenases.