6533b861fe1ef96bd12c591d

RESEARCH PRODUCT

Apparent transinhibition of peptide uptake in the scutellum of barley grain

subject

description

The uptake of glycylsarcosine (Gly‐Sar) into scutella separated from germinating grains of barley (Hordeum vulgare L. cv. Himalaya) is inhibited by other peptides; in most cases the inhibition is not purely competitive but of a mixed type (simultaneous increase in the apparent Km and decrease in Vmax) (Sopanen, T. 1979. FEBS Lett. 108: 447–450). The aim of the present experiments was to elucidate the mechanism of the mixed inhibition by studying how peptides already taken up into the cells affect the uptake of Gly‐Sar. When scutella were preincubated in the presence of various peptides, 11 of the 13 peptides tested inhibited the subsequent uptake of Gly‐Sar by 10 to 45%. The inhibition, studied in detail with leucylleucine and prolylproline, was due to a decrease in Vmax. The two peptides having no effect were glycylglycine and D‐alanyl‐L‐alanine which are the only peptides known to date acting as purely competitive inhibitors when present together with the substrate Gly‐Sar. Preincubation with leucine, proline and alanine was not inhibitory, although preincubation with the corresponding dipeptides was. This result, together with the demonstration of intact leucylleucine in the scutella after preincubation with leucylleucine, indicates that the inhibition was caused by the intact peptides. The results support the notion that in the mixed type inhibition the increase in the apparent Km is due to competition for the carrier at the outside of the membrane, while the decrease in Vmax is due to peptides taken up and binding to the carrier at the inside of the membrane.