EVIDENCE FOR INTRACELLULAR PARTITIONING OF SERINE AND GLYCINE METABOLISM IN CHINESE-HAMSTER OVARY CELLS

Serine hydroxymethyltransferase (SHMT) is the primary enzyme in the in terconversion of serine and glycine. The roles of mitochondrial and cy tosolic SHMT in the interconversion of serine and glycine were determi ned in two Chinese hamster ovary (CHO) cell lines that both contain cy tosolic SHMT but either have (CHOm(+)) or lack (CHOm(-)) mitochondrial SHMT. Mitochondrial SHMT activity was significantly reduced in CHOm(- )(0.24+/-0.11 nmol/min per mg of mitochondrial protein) compared with CHOm(+) (3.21+/-0.66 nmol/min per mg of mitochondrial protein; P = 0.0 2) cells, whereas cytosolic SHMT activity was similar in CHOm(-) and C HOm(+) cells (1.09+/-0.31 and 1.53 +/- 0.12 nmol/min per mg of cytosol ic protein respectively; P = 0.57). In CHOm(+) and CHOm(-) cells, the relative flux of glycine to serine measured with either [1-C-13]- or [ 2-C-13]-glycine was similar (CHOm(-): 538 +/- 82 nmol/24 per mg of DNA ; CHOm(+): 616+/-88 nmol/24 h per mg of DNA; P = 0.42). In contrast, t he relative flux of serine to glycine measured with [1-C-13]serine was low in CHOm(-) cells (80+/-28 nmol/24h per mg of DNA) compared with C HOm(+) cells (3080+/-320 nmol/24 h per mg of DNA; P = 0.0001). The rat e of glycine production determined by [1-C-13]glycine dilution was low er in CHOm(-) (1200+/-200 nmol/24 h per mg of DNA) than CHOm(+) (10200 +/- 1800 nmol/24 h per mg of DNA; P = 0.03) cells, whereas glycine ut ilization was similar in the two cell lines. Serine production was sim ilar in the two cell lines but serine utilization was lower in CHOm(-) (3800+/-1200 mu mol/24h per mg of DNA) than CHOm(+) (6600 +/- 1000 nm ol/24 h per mg of DNA; P = 0.0002) cells. Increasing the serine concen tration in the medium resulted in an increase in glycine production in CHOm(+) but not in CHOm(-) cells. Intracellular studies with [1-C-13] serine confirm the findings of decreased glycine production from serin e. In CHO cells there is partitioning of intracellular serine and glyc ine metabolism, Our data support the hypothesis that mitochondrial SHM T is the primary pathway for serine into glycine interconversion.