Mechanism of nitrate reductase reversible inactivation by ammonia in Chlamydomonas

Abstract

Ammonia promotes in vivo the conversion of the active form of nitrate reductase into its inactive form by indirectly causing the reduction of the enzyme. Apparently ammonia acts as an uncoupler of noncyclic photophosphorylation, a process which in turn leads to a rise in the level of reducing power in the cell. The transformation is reversible, and, upon ammonia removal, the enzyme becomes again oxidized and active. Nitrate reductase inactivation by ammonia requires light and does not occur when the noncyclic electron flow is blocked or when the photosynthetically generated reducing power is being simultaneously oxidized. In the dark and in the absence of ammonia, the enzyme appears either active or inactive in response to the degree of aerobicity or anaerobicity of the culture.