Identification of an l-serine/l-threonine dehydratase with glutamate racemase activity in mammals

Abstract

Recent investigations have shown that multiple D-amino acids are present in mammals and these compounds have distinctive physiological functions. Free D-glutamate is present in various mammalian tissues and cells and in particular itis presumably correlated with cardiac function, and much interest is growing in its unique metabolic pathways. Recently, we first identified D-glutamate cyclase as its degradative enzyme in mammals, whereas its biosynthetic pathway in mammals is unclear. Glutamate racemase is a most probable candidate, which catalyzes interconversion between D-glutamate and L-glutamate. Here, we identified the cDNA encoding L-serine dehydratase-like (SDHL) as the first mammalian clone with glutamate racemase activity. This rat SDHL had been deposited in mammalian databases as a protein of unknown function and its amino acid sequence shares approximately 60% identity with that of L-serine dehydratase. Rat SDHL was expressed in Escherichia coli, and the enzymatic properties of the recombinant were characterized. The results indicated that rat SDHL is a multifunctional enzyme with glutamate racemase activity in addition to L-serine/L-threonine dehydratase activity. This clone is hence abbreviated as STDHgr. Further experiments using cultured mammalian cells confirmed that D-glutamate was synthesized and L-serine and L-threonine were decomposed. It was also found that SDHL (STDHgr) contributes to homeostasis of several other amino acids.

Glyoxylate reductase/hydroxypyruvate reductase regulates the free d‐aspartate level in mammalian cells

“…On the other hand, we previously demonstrated that DDO mRNA is barely detectable in 293 cells. 36 Together, these findings imply that the inhibitory effect of D-Asp against the growth of 293 cells (293/NEO cells) is independent of the metabolism by DDO of D-Asp taken up by the cells; the results in Figure 5A indicate that 293 cells are capable of taking up D-Asp. Indeed, the LD 50 value of D-Asp in cells overexpressing full-length (PTS-containing) DDO (293-hDDO-1 cells) was comparable to that in 293/NEO cells (Figure 1B).…”

Identifying sources of D-serine in Caenorhabditis elegans and their impact on behavior

“…This insolubility, in combination with the general lack of native post-translational modifications in bacterial expression systems, may contribute to the lack of enzymatic activities observed in the Serine/threonine dehydratase (aka serine/threonine ammonia-lyase) activity of recombinant proteins was also measured to check whether the recombinant proteins are in active forms (Table 2). Serine/threonine dehydratases (ammonia-lyases) catalyze the conversion of L-Ser/L-threonine (L-Thr) into 2-oxo acid products and ammonia (15). As expected, SERR-1 displayed serine dehydratase activity, although about an order or magnitude lower than reported (13).…”

Promiscuous enzymes generating d-amino acids in mammals: Why they may still surprise us?

“…SDHL normally catalyzes the α,β-elimination of water from L-serine and L-threonine ( Figure 1). Although glutamate racemization efficiency is almost three orders of magnitude lower than the α,β-elimination reactions, D-glutamate accumulates in mammalian cells overexpressing the SDHL gene [12].…”

“…Except for D-serine [9][10][11], their role and origin remain elusive. A new study by Homma and colleagues [12] indicates that enzyme promiscuity underlies some D-amino acid production in mammals. Mammals do not possess canonical glutamate racemases, but D-glutamate is present in the heart [8] and accounts for about 20% of total glutamate in human urine [7].…”

“…Homma and colleagues now report that promiscuous glutamate racemization by the serine/threonine dehydratase-like (SDHL) enzyme is responsible for D-glutamate accumulation in mammals [12]. SDHL normally catalyzes the α,β-elimination of water from L-serine and L-threonine ( Figure 1).…”