Uptake of ornithine by rat liver mitochondria

Uptake of [14C]-L-ornithine by rat liver mitochondria has been measured by using the silicone sampling technique. The uptake of ornithine measured after 20-45 s of incubation exhibits stereospecificity, pH dependence, and a lack of dependence on respiratory energy. A slower subsequent increase in [14C]-L-ornithine counts associated with the mitochondria, which is blocked by the transaminase inhibitor aminooxyacetate, is attributed to metabolism of the labeled ornithine. Each of the reagents N-ethylmaleimide, Tris (HCl) buffer, Tl2+SO42(-), Mg2+SO42(-), and choline chloride inhibits ornithine accumulation. A lack of inhibition by mersalyl is interpreted as indicating that ornithine uptake does not require transmembrane Pi flux. Uptake of ornithine to levels in excess of the concentration in the medium can largely be accounted for by an osmotically insensitive fraction of the ornithine taken up, which is assumed to be adsorbed to solid structures of the mitochondria.