Aminobutyrate by-pass system in Escherichia coli

Abstract

The enzymatic activities related to the 4-aminobutyrate (4-AB) by-pass system, i. e. glutamate decarboxylase (I), 4-AB: 2-oxoglutarate aminotransferase (II), succinic semialdehyde dehydrogenase (III) and glutamate dehydrogenase (IV), have been stu­ died in E. coli grown in different culture media. As far as we know, activities II and III are shown to occur in E. coli for the first time. When 4-AB or glutamate were used as the only nitrogen source activities I, II and III increased and, with the latter, IV de­ creased. Results suggest the occurrence of two constitutive enzymes (I and III) which activity is enhanced or depressed according to the growth conditions, and the transami­ nase, of inducible nature, which biosynthesis would be favoured when the amination through glutamate dehydrogenase cannot take place with the required efficiency. Re­ markable variations were found in the activity of I according to the media when pyri- doxal-5-phosphate was added, showing different degrees of apoenzyme saturation in the assayed conditions. When sonicated extracts were used instead of acetone powders, higher absolute values of the activities were obtained. No significant short-term effects were found when the cells were tranferred from the media with inorganic N plus glu­ cose to media with 2-oxoglutarate, succinate, 4-aminobutyrate o 4-hydroxibutyrate. Only glutamate showed short-tem induction of I after 60 min. in a culture of 4 hr. The effects obtained after different periods of growth (4, 12, 16 and 22 hors) show its influence on the functioning of the system. Using several metabolites as the sole C source, succinate, acetate and especially malate and pyruvate depressed I activity. Endo- cellular 4-AB is high in ammonia plus glucose media, being released. When 4-AB is used as N source, 2-oxoglutarate accumulates and is excreted to the media.