Glycine homeostasis requires reverse SHMT flux

Abstract

The folate-dependent enzyme serine hydroxymethyltransferase (SHMT) reversibly converts serine into glycine and a tetrahydrofolate-bound one-carbon unit. Such one-carbon unit production plays a critical role in development, the immune system, and cancer. Here we show that the whole-body SHMT flux acts to net consume rather than produce glycine. Pharmacological inhibition of whole-body SHMT1/2 and genetic knockout of liver SHMT2 elevated circulating glycine levels up to eight-fold. Stable isotope tracing revealed that the liver converts glycine to serine, which is then converted by serine dehydratase into pyruvate and burned in the tricarboxylic acid cycle. In response to diets deficient in serine and glycine, de novo biosynthetic flux was unaltered but SHMT2- and serine dehydratase-mediated catabolic flux was lower. Thus, glucose-derived serine synthesis does not respond to systemic demand. Instead, circulating serine and glycine homeostasis is maintained through variable consumption, with liver SHMT2 as a major glycine-consuming enzyme.

Cycling back to folate metabolism in cancer

“…However, the principal role of the GCS appears to be to limit glycine accumulation, with germline mutations in GLDC (a major component of the GCS) resulting in glycine encephalopathy 26 . Most recently, the 1C units produced by GCS have been suggested to drive reverse flux of SHMT2, producing serine and clearing excess mitochondrial glycine in the liver 27 . Choline contributes 1C units to support both the methionine cycle and – through the production of sarcosine – the mitochondrial folate cycle.…”

“…Most recently, the 1C units produced by GCS have been suggested to drive reverse flux of SHMT2 to clear excess mitochondrial glycine in the healthy liver. This regulation of homeostatic glycine levels could be an important factor when dietary depletion of serine is considered for cancer therapy 27 .…”