Major Pathways of Serine and Glycine Catabolism in Various Organs of the Rat and Cock*

Abstract

The metabolism of serine and glycine in the liver, kidney, spleen, testis, brain, spinal cord, heart, lung, skeletal muscle, and small intestine of rats and cocks was studied with whole tissue homogenates, soluble tissue fractions, and mitochondria. The soluble tissue fractions of all organs tested contained serine hydroxymethyl-transferase [EC 2.1.2.1] and methylene-THF dehydrogenase [EC 1.5.1.5]. On the other hand, the distribution of L-serine dehydratase [EC 4.2.1.13] was very limited; only rat liver and kidney contained significant activity and the activity in the latter being very low. Methylene-THF, formed from the β-carbon of serine, can be further oxidized to CO2 either by soluble fractions or mitochondria of most rat organs. This may be due to the sequential actions of methylene-THF dehydrogenase, cyclohydrolase [EC 3.5.4.9], and 10-formyl-THF: NADP+ oxidoreductase. In contrast, preparations from various organs of cocks caused no appreciable catalysis of the oxidation of the β-carbon of serine or formaldehyde to CO2. This is probably because the activity of 10-formyl-THF: NADP+ oxidoreductase is extremely low in these organs. It was also found that the glycine cleavage system is important in glycine degradation in the liver, kidney, brain, and testis of both rats and cocks. All other organs from these animals except heart caused scarcely any catabolism of glycine to CO2. In heart there was considerable glycine catabolism, which did not appear to occur by either the glycine cleavage or the glyoxylate pathway. It was concluded that serine catabolism in rats and cocks can be accounted for mainly by the cleavage of serine to form methylene-THF and glycine, followed by the cleavage of glycine to methylene-THF, CO2, and ammonia, and that this is also the major pathway of glycine degradation in both rats and cocks.