Selective regulatory effects of purine and pyrimidine nucleotides on vacuolar transport of amino acids

Reconstitution into liposomes of the glutamine/amino acid transporter from renal cell plasma membrane: functional characterization, kinetics and activation by nucleotides

The glutamine/amino acid transporter was solubilized from rat renal apical plasma membrane (brush-border membrane) with C12E8 and reconstituted into liposomes by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite XAD-4. The reconstitution was optimised with respect to the protein concentration, the detergent/phospholipid ratio and the number of passages through a single Amberlite column. The reconstituted glutamine/amino acid transporter catalysed a first-order antiport reaction stimulated by external, not internal, Na+. Optimal activity was found at pH 7.0. The sulfhydryl reagents HgCl2, mersalyl and p-hydroxymercuribenzoate and the amino acids alanine, serine, threonine, cysteine, asparagine, methionine and valine strongly inhibited the transport, whereas the amino acid analogue methylaminoisobutyrate had no effect. Glutamine, alanine, serine, asparagine, threonine were efficiently translocated from outside to inside and from inside to outside the proteoliposomes as well. Cysteine and valine were translocated preferentially from outside to inside. The Km for glutamine on the external and internal side of the transporter was 0.47 and 11 mM, respectively; the values were not influenced by the type of the counter substrate. The transporter is functionally asymmetrical and it is unidirectionally inserted into the proteoliposomal membrane with an orientation corresponding to that of the native membrane. By a bisubstrate kinetic analysis of the glutamine antiport, a random simultaneous mechanism was found. The glutamine antiport was strongly stimulated by internal nucleoside triphosphates and, to a lower extent, by pyrophoshate. The reconstituted glutamine/amino acid transporter functionally corresponds to the ASCT2 protein.

Uric acid is a genuine metabolite of Penicillium cyclopium and stimulates the expression of alkaloid biosynthesis in this fungus

On searching for endogenous, low-molecular-weight effectors of benzodiazepine alkaloid biosynthesis in Penicillium cyclopium uric acid was isolated from ethanolic or autoclaved mycelial extracts of this fungus. The isolation was based on a three-step high-pressure liquid chromatography procedure guided by a microplate bioassay, and uric acid was identified by mass spectrometry and the uricase reaction. Conidiospore suspensions that were treated with this compound during the early phase of outgrowth developed emerged cultures with an enhanced rate of alkaloid production. Uric acid treatment did not increase the in vitro measurable activity of the rate-limiting biosynthetic enzyme, cyclopeptine synthetase. However, these cultures displayed a reduced rate of uptake of the alkaloid precursor L-phenylalanine into the vacuoles of the hyphal cells as assayed in situ. It is suggested that the depressed capacity of vacuolar uptake caused by the contact of outgrowing spores with uric acid liberated from hyphal cells results in an enhanced availability of the precursor L-phenylalanine in the cytoplasm and thus accounts at least in part for the increase in alkaloid production.