Glycine turnover and decarboxylation rate quantified in healthy men and women using primed, constant infusions of [1,2-(13)C2]glycine and [(2)H3]leucine

Abstract

Glycine plays several roles in human metabolism, e.g. as a 1-carbon donor, in purine synthesis, and as a component of glutathione. Glycine is decarboxylated via the glycine cleavage system (GCS) that yields concurrent generation of a 1-carbon unit as 5,10-methylenetetrahydrofolate (methyleneTHF). Serine hydroxymethyltransferase (SHMT) catalyzes the interconversion of glycine and serine, another 1-carbon donor. The quantitative role of glycine in human 1-carbon metabolism has received little attention. The aim of this protocol was to quantify whole body glycine flux, glycine to serine flux, and rate of glycine cleavage in humans. A primed, constant infusion with 9.26 micromol x kg(-1) x h(-1) [1,2-(13)C2]glycine and 1.87 micromol x kg(-1) x h(-1) [(2)H3]leucine was used to quantify the kinetic behavior of glycine in young, healthy volunteers (n = 5) in a fed state. The isotopic enrichment of infused tracers and metabolic products in plasma, as well as breath (13)CO2 enrichment, were determined for use in kinetic analysis. Serine synthesis by direct conversion from glycine via SHMT occurred at 193 +/- 28 micromol x kg(-1) x h(-1) (mean +/- SEM), which comprised 41% of the 463 +/- 55 micromol x kg(-1) x h(-1) total glycine flux. Nearly one-half (46%) of the glycine-to-serine conversion occurred using GCS-derived methyleneTHF 1-carbon units. Based on breath (13)CO2 measurement, glycine decarboxylation (190 +/- 41 micromol x kg(-1) x h(-1)) accounted for 39 +/- 6% of whole body glycine flux. This study is the first to our knowledge to quantify human glycine cleavage and glycine-to-serine SHMT kinetics. GCS is responsible for a substantial proportion of whole body glycine flux and constitutes a major route for the generation of 1-carbon units.

Figure 1

Schematic of carbon flow from infused [¹³C₂]glycine to give singly or doubly labeled serine. This is the basis of differentiating whether labeled serine is generated directly from glycine via SHMT (lower pathway yielding [¹³C₂]serine) or via SHMT using a glycine-derived methyleneTHF (upper pathway yielding [¹³C₁]serine).

Figure 2

[¹³C₂]Glycine tracer infusion protocol.

Figure 3

Plasma enrichment of infused amino acids (A) and metabolic products (B) in healthy men and women during primed, constant infusion with 9.26 μmol/(kg·h) [¹³C₂]glycine and 1.87 μmol/(kg·h) [²H₃]leucine. Values are means ± SEM, n=5.