Summary of project PR000626

This data is available at the NIH Common Fund's National Metabolomics Data Repository (NMDR) website, the Metabolomics Workbench, https://www.metabolomicsworkbench.org, where it has been assigned Project ID PR000626. The data can be accessed directly via it's Project DOI: 10.21228/M8RH5S This work is supported by NIH grant, U2C- DK119886.

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Project ID:	PR000626
Project DOI:	doi: 10.21228/M8RH5S
Project Title:	Evidence that the metabolite repair enzyme NAD(P)HX epimerase has a moonlighting function
Project Summary:	Damaging enzymatic and chemical side-reactions in all organisms convert NADH and NADPH to their hydrates, NADHX and NADPHX, which exist as R and S epimers [1,2]. The hydrates, which inhibit various dehydrogenases [3,4], are reconverted to NAD(P)H by the sequential actions of NADP-(H)X epimerase (EC 5.1.99.6) and NAD(P)HX dehydratase (EC 4.2.1.93) [5-7] (Figure 1A). Both enzymes occur in all domains of life [5]. The dehydratase is specific for the S form of NAD(P)HX [1] and the epimerase facilitates conversion of the R form to the S form used by the dehydratase [5-7]. Over time, if not reconverted to NAD(P)H, the R and S forms of NAD(P)HX give rise spontaneously to cyclic forms of NAD(P)HX, which are not substrates for the epimerase or the dehydratase [1,5] (Figure 1A). Together, the formation and removal of NAD(P)HX constitute an archetypal example of metabolite damage and repair [8-10]. The functions of the epimerase and the dehydratase are supported by biochemical evidence from mammals, yeast, Escherichia coli, and plants [5-7], and that of the dehydratase is also supported by genetic and metabolomic evidence from Arabidopsis [6]. The in-vivo function of the NAD(P)HX dehydratase protein seems to match its in-vitro enzymatic activity [6], but the situation for the epimerase protein may be more complex, as several discordant ob3 servations suggest. First, as NAD(P)HX epimers equilibrate spontaneously and quite rapidly (t½ = 40 min) in physiological conditions, it is not entirely clear why epimerase activity is needed [1]. Second, while the epimerase is fused to the dehydratase in most prokaryotes, it is fused in plants to pyridoxine/pyridoxamine phosphate oxidase (PPOX), a salvage enzyme for vitamin B6, i.e. pyridoxal 5′-phosphate (PLP) and related compounds [5-7,11,12]. Since protein fusions imply functional relationshipsbetween the partners [13,14], the PPOX fusion points to a B6 connection, in plants at least. Third, and relatedly, epimerase and dehydratase expression are not correlated in Arabidopsis [7]. Fourth, the mammalian epimerase (NAXE) occurs in extracellular compartments (cerebrospinal fluid, urine, and the plasma of septic patients) [15], which the dehydratase does not – although both enzymes occur in the mitochondria and cytosol [16]. Moreover, NAXE can bind apolipoprotein A-1, a plasma protein that is a major component of the high-density lipoprotein (HDL) complex [15]. This binding is proposed to link NAXE to cholesterol transport, atherosclerosis, and angiogenesis [17,18]. To summarize the above, there are reasons to suspect that NAD(P)HX epimerase is more than just an epimerase, and that it has a moonlighting function [19] related to B6. This suspicion has become medically relevant with the discovery that nonsense or missense mutations in the human epimerasegene NAXE (also called AIBP) lead to a neurodegenerative disease that is lethal in infancy [20, 21]. The etiology of this disease – and potential therapies – have so far been conceived solely in terms of loss of NAD(P)HX epimerase activity [20,21]. While the NAXE protein deficiency and cyclic NADHX accumulation observed in fibroblasts from affected individuals [21] are consistent with lack of epimeraseactivity causing the symptoms, they do not prove that this is the case. Nor is it evident why intracellular buildup of NADHX to the reported level of ~5 pmol/mg protein [21], equivalent to ~1 μM [22], would have such a severe impact in humans given that a similar accumulation in Arabidopsis had no discernable consequences [6] and that 1 μM NADHX corresponds to only ~1% of the NADH pool in typical mammalian cells [23]. These considerations led us to use comparative genomic and genetic approaches to explore the possibility that NAD(P)HX epimerase has a moonlighting function. Our results build a strong circumstantial case that such a function exists, and that it involves B6 metabolism.
Institute:	University of Florida
Department:	Horticultural Sciences
Last Name:	Hanson
First Name:	Andrew
Address:	Gainesville, FL 32611
Email:	adha@ufl.edu
Phone:	352-273-4856

Summary of all studies in project PR000626

Study ID	Study Title	Species	Institute	Analysis (* : Contains Untargted data)	Release Date	Version	Samples	Download (* : Contains raw data)
ST000900	Evidence that the metabolite repair enzyme NAD(P)HX epimerase has a moonlighting function	Escherichia coli	University of California, Davis	MS*	2017-11-20	1	18	Uploaded data (94.1M)