부산대학교 도서관

ε-Crystallin occurs as an abundant lens protein in many birds and in crocodiles and has been identified as heart-type lactate dehydrogenase (LDH-B4). Lens proteins have, due to their longevity and environmental conditions, extraordinary requirements for structural stability. To study lens-protein stability, we compared various parameters of LDH-B4/ε-crystallin from lens and/or heart of duck, which has abundant amounts of this enzyme in its lenses, and of chicken and pig, which have no ε-crystallin. Measuring the thermostability of LDH-B4 from the different sources, the t[sub 50] values (temperature at which 50% of the enzyme activity remains after a 20-min period) for LDH-B4 from duck heart, duck lens and chicken heart were all found to be around 76°C, whereas pig heart LDH-B4 was less thermostable, having a t[sub 50] value of 62.5°C. A similar tendency was found with urea inactivation studies. Plotting the first-order rate constants obtained from inactivation kinetic plots against urea concentration, it was clear that LDH-B4 from pig heart was less stable in urea than the homologous enzymes from duck heart, chicken heart and duck lens. The duck and chicken enzymes were also much more resistant against proteolysis than the porcine enzyme. Therefore, it is concluded that avian LDH-B4 is structurely more stable than the homologous enzyme in mammals. This greater stability might make it suitable to function as a crystallin, as in duck, but is not necessarily associated with high lens expression, as in chicken. [ABSTRACT FROM AUTHOR]