Similarity and differences in the physicochemical properties of lactate dehydrogenase isozymes from different tissues of Japanese sandfish Arctoscopus japonicus

Vertebrates have three genes ( LDHA , LDHB , and LDHC ) encoding skeletal muscle-, heart muscle- and testis-specific lactate dehydrogenase (LDH, E.C. 1.1.1.27), respectively. The muscle and heart LDHs are tetramers formed by polypeptides encoded by LDHA and LDHB, but not LDHC. The catalytic activity and physicochemical characteristics of testis LDH (tLDH) differs from those of the other two isozymes. There have been few kinetic analyses of fish tLDH. Moreover, the mechanism enabling enzymatic activity to be sustained in low temperature-adapted fish remains unclear. In the present study, tLDH and white muscle LDH (mLDH) were isolated from the Japanese sandfish Arctoscopus japonicas, the habitat of which ranges from 1.5 to 13 °C in temperature. The K m and V max of mLDH and tLDH with pyruvate were similar, but their thermostabilities differed. The of K m and V max values increased with increasing temperature between 5 and 40 °C, and the van’t Hoff Δ H values for pyruvate reduction by mLDH and tLDH were 35 and 31 kJ mol −1 , respectively. Our findings indicate that LDH function and structure (thermal stability) are highly conserved in skeletal muscle and testis, but unique properties are acquired in each tissue depending on its function.