Effect of heat treatment on the activity in vitro of nitrate reductase from Cyanidium caldarium

Abstract

Nitrate reductase from the unicellular alga Cyanidium caldarium is present partly in a fully active form and partly in a latent form. The latent nitrate reductase becomes expressed at o°C: the reactivation of enzyme is dependent on the ionic strength of the buffer. It is expressed also upon heat treatment above 42°C; however, this heat-activated nitrate reductase can utilise only reduced benzyl viologen and not NADPH as electron donor for nitrate reduction. The ΔH a for conversion from latent into active benzyl viologen nitrate reductase is 69.5 kcal. When fully active nitrate reductase is heated at temperatures above 42°C, a loss of the NADPH-dependent activity occurs; the ΔH a for this process is 68.5 kcal. Inactivation of the benzyl viologen-dependent activity occurs at temperatures above 64°C with an ΔH a of 114 kcal. The Arrhenius plot exhibits a break at 24°C; above this temperature the energies of activation calculated are: benzyl viologen, 6.15 kcal; NADPH, 7.55 kcal.