ALDH1L1, a common enzyme in folate metabolism, converts 10-formyltetrahydrofolate (10-formyl-THF) to tetrahydrofolate (THF). This reaction is known to regulate the de novo purine biosynthesis and folate-dependent homocysteine re-methylation cycle. It could also play a key role in controlling the flux of one-carbon groups to anabolic pathways. In support of the role in the regulation of folate metabolism, we have recently shown that the loss of the ALDH1L1 gene in knockout mice causes functional folate deficiency, even when the mice had sufficient folate intake. Analysis of Aldh1l1 knockout mice also showed that the enzyme is the main regulator of glycine metabolism in the liver: KO mice have lower levels of glycine and glycine conjugates, indicating that the enzyme is involved in the folate-dependent synthesis of glycine from serine. We have further reported that human ALDH1L1 has six common non-synonymous exonic SNPs at the polymorphic loci rs3796191, rs2886059, rs9282691, rs2276724, rs1127717 and rs4646750 with the occurrence of haplotypes associated with these SNPs being remarkably different between ethnic populations. Our analysis of an established cohort of Hispanic children, Viva La Familia, has shown a significant reduction of serum glycine and increase in serine/glycine ratio in children with rs2276724 and rs3796191, indicating deregulation of serine to glycine conversion and re- capitulating our mouse findings. ALDH1L1 non-synonymous SNPs were also associated with markers of metabolic stress and adiposity in this cohort. Based on our findings, we hypothesize that non-synonymous ALDH1L1 SNPs produce enzyme variants with altered catalytic activity and/or stability that affects their ability to metabolize 10-formyl-THF and thus deregulates glycine metabolism. Accordingly, individuals with specific haplotypes have different ratios of THF/10-formyl-THF and serine/glycine, and altered levels of glycine and its conjugates, with perturbations in the metabotype representing a signature of metabolic health. This proposal will address the question of how haplotype-specific ALDH1L1 variants affect folate metabolism and the overall cellular metabotype, and how the haplotype-specific effect is modified by folate supplementation, by pursuing the following specific aims. Aim 1. Functionally characterize the ALDH1L1 enzyme variants from common human haplotypes. Aim 2. Determine the impact of major ALDH1L1 haplotypes on cellular metabolism and haplotype- specific responses to various folate supplementations. Aim 3. Link ALDH1L1 haplotypes to the folate-dependent regulation of glycine metabolism and health outcomes in humans. ALDH1L1 variants are very common in different populations but their role in folate homeostasis and in the etiology of metabolic disease is largely unexplored. It is expected that ALDH1L1 haplotypes differently mediate the metabolic response to dietary folate that might require adjustments of folate intake for individuals bearing certa...