The present report identifies the enzymatic substrates of a member of the mammalian
nitrilase-like (Nit) family. Nit2, which is widely distributed in nature, has been suggested to be a
tumor suppressor protein. The
protein was assumed to be an
amidase based on sequence homology to other
amidases and on the presence of a putative
amidase-like active site. This assumption was recently confirmed by the publication of the crystal structure of mouse Nit2. However, the in vivo substrates were not previously identified. Here we report that rat liver Nit2 is omega-amidodicarboxylate
amidohydrolase (E.C. 3.5.1.3; abbreviated
omega-amidase), a ubiquitously expressed
enzyme that catalyzes a variety of
amidase, transamidase,
esterase and transesterification reactions. The in vivo
amidase substrates are
alpha-ketoglutaramate and alpha-ketosuccinamate, generated by transamination of
glutamine and
asparagine, respectively.
Glutamine transaminases serve to salvage a number of alpha-
keto acids generated through non-specific transamination reactions (particularly those of the
essential amino acids).
Asparagine transamination appears to be useful in mitochondrial metabolism and in photorespiration.
Glutamine transaminases play a particularly important role in transaminating
alpha-keto-gamma-methiolbutyrate, a key component of the
methionine salvage pathway. Some evidence suggests that excess
alpha-ketoglutaramate may be neurotoxic. Moreover, alpha-ketosuccinamate is unstable and is readily converted to a number of hetero-aromatic compounds that may be toxic. Thus, an important role of
omega-amidase is to remove potentially toxic intermediates by converting
alpha-ketoglutaramate and alpha-ketosuccinamate to biologically useful
alpha-ketoglutarate and
oxaloacetate, respectively. Despite its importance in
nitrogen and
sulfur metabolism, the biochemical significance of
omega-amidase has been largely overlooked. Our report may provide clues regarding the nature of the
biological amidase substrate(s) of Nit1 (another member of the Nit family), which is a well-established
tumor suppressor protein), and emphasizes a) the crucial role of Nit2 in
nitrogen and
sulfur metabolism, and b) the possible link of Nit2 to
cancer biology.