Nitric oxide synthase activity in mitochondria

TLDR

NOS catalyzes the conversion of l‑arginine to nitric oxide and l‑citrulline in a Ca²⁺‑dependent manner. The study demonstrates the presence of a functional nitric oxide synthase in rat liver mitochondria and proposes that its Ca²⁺ dependence is crucial for mitochondrial function. Mitochondrial NOS is localized to the inner membrane, is constitutively active in intact mitochondria, regulates respiration and membrane potential, is stimulated by Ca²⁺ uptake, and is inhibited by l‑arginine analogues while d‑arginine is not a substrate.

Abstract

In the present study we show the existence of a functional nitric oxide synthase (NOS) in rat liver mitochondria. The enzyme uses l ‐arginine ( l ‐arg) to produce nitric oxide (NO) and l ‐citrulline, and is Ca 2+ ‐dependent. l ‐Arg analogues, N ω ‐monomethyl‐ l ‐arg and N ω ‐nitro‐ l ‐arg, inhibit the enzyme, and d ‐arginine is not a substrate for it. We found mitochondrial NOS (mtNOS) activity associated with the inner mitochondrial membrane but not with the matrix fraction. In intact, succinate‐energized mitochondria, the enzyme is constitutively active and exerts substantial control over mitochondrial respiration and membrane potential. The activity is further stimulated when Ca 2+ is taken up by mitochondria. We suggest that the existence of mtNOS and its Ca 2+ dependence are highly relevant for mitochondrial functioning.