Purification and characterization of NADH-dependent glutamate synthase from the silkworm fat body (Bombyx mori).
NADH-dependent glutamate synthase (Nadh-Gogat; EC 1.41.14) was purified 766-fold from the fat body of 5th instar larvae of the silkworm with a final specific activity of 13.8 units/mg protein by a produce including ammonium sulfate fraction, Q-Sepharose HP ion exchange column chromatography, Blue Sepharose FF affinity column chromatography and Superdex 200 HR gel filtration. The purified enzyme yielded a single band corresponding to a molecular mass of 195kDa by SDS-polyacrylamide gel electrophoresis. Molecular mass of the native enzyme was estimated to be 190 kDa by Superdex 200HR gel filtration, suggesting that the enzyme is composed of a monomeric polypeptide. The enzyme showed an absorption spectrum with maximum values at 272, 375, and 435 nm, suggesting the presence of a flavin prosthetic group in the enzyme. The N-terminal amino acid sequence showed a high similarity to those of other GOGATs from plants, yeast and bacteria, but no similarity to other known proteins was detected. The enzyme exhibited a strong specificity to the electron donor and substrates; NADH as electron donor, 2-oxoglutarate as amino acceptor and glutamine as amino donor were essential for the catalytic activity. The optimum pH was around 7.5, at which Km values for 2-oxoglutarate, glutamine and NADH were 17, 220 and 5.7 micro M, respectively. Azaserine, 6-diazo-5-oxonorleucine and p-chloromercuribenzoic acid were strong inhibitors of the activity. These result show that NADH-GOGAT in the silkworm fat body strongly resembles other eukaryotic NADH-GOGATs, suggesting that it plays a significant role in ammonia assimilation in the same manner as the other enzymes.
National Institute of Sericultural and Entomological Science, Ibaraki, Japan. Hirayam@nises.affrc.go.jp
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D000595:Amino Acid Sequence; D000818:Animals; D012831:Bombyx / Q000201:enzymology*; D004583:Electrons; D005216:Fat Body; D005970:Glutamate Synthase / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009243:NAD / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013056:Spectrophotometry, Ultraviolet; D013379:Substrate Specificity
D009243:NAD; D005970:Glutamate Synthase