Eleven years of experience in fes assisted gait of spinal cord injured patients

Abstract

Penicillium chrysogenum possesses a highly specific sulfur-regulated permease for l-methionine (Benko, Wood, and Segel, Arch. Biochem. Biophys.122, 783–804 (1967)). In addition, the mycelium has several other relatively specific amino acid permeases. For example, l-phenylalanine-14C transport by sulfur- and nitrogen-sufficient mycelium is not significantly inhibited by a 10-fold excess of unlabeled l-methionine, l-leucine, l-serine, or l-α-aminobutyric acid. Similarly, l-leucine-14C transport is not significantly inhibited by a 10-fold excess of any of the other four amino acids. Nitrogen-starvation results in the development of a nonspecific amino acid permease (Vmax for l-methionine, l-leucine, and l-phenylalanine ca. 10 μmoles/g-min). The development is prevented by actidione. Structural requirements for interaction with the nonspecific amino acid permease are: (a) an unsubstituted l-α-amino group, (b) a single α-hydrogen atom, (c) no secondary negatively charged group, and (d) an α-carbonyl group. The Km values of the nonspecific permease for l-phenylalanine and l-methionine are ca. 10−5m. The Ki for l-leucine (with l-methionine as substrate) is 2.4 × 10−5m; the Ki for l-methionine (with l-leucine as substrate) is 1.4 × 10−5m. Other characteristics of the permease are pH-dependence (optimum at 6), temperature-dependence (Q10 of 2–5 between 15 and 32 °), and energy-dependence (DNP and azide sensitive). The general amino acid permease is independent of monovalent and divalent metal ions and of concurrent protein synthesis for activity. Ammonia (NH4+) was the only non-amino acid inhibitor of the permease. The permease activity can be markedly and rapidly reduced by preloading the nitrogen-deficient mycelium with NH4+ or any one of several amino acids. The results suggest that the nonspecific amino acid permease is subject to feedback regulation by intracellular NH4+ and amino acid substrates.