A view of the comparison of the regulation of enzymes in mammalian and microbial systems

The importance of serine metabolism in cancer

Serine metabolism is frequently dysregulated in cancers; however, the benefit that this confers to tumors remains controversial. In many cases, extracellular serine alone is sufficient to support cancer cell proliferation, whereas some cancer cells increase serine synthesis from glucose and require de novo serine synthesis even in the presence of abundant extracellular serine. Recent studies cast new light on the role of serine metabolism in cancer, suggesting that active serine synthesis might be required to facilitate amino acid transport, nucleotide synthesis, folate metabolism, and redox homeostasis in a manner that impacts cancer.

Histochemical technique for the demonstration of pyruvate kinase activity

We describe an enzyme histochemical multistep technique for the demonstration of pyruvate kinase activity. In this technique, a semipermeable membrane is interposed between the incubation medium and the tissue sections, thus preventing diffusion of the enzyme into the medium during the incubation period. In this histochemical system, phosphoenolpyruvate (PEP) donates its phosphate group to ADP in a reaction catalysed by pyruvate kinase. Next, exogenous and endogenous hexokinase catalyses the reaction between ATP and D-glucose to yield D-glucose-6-phosphate and ADP. The D-glucose-6-phosphate is oxidized by exogenous and endogenous D-glucose-6-phosphate dehydrogenase, and concomitantly, the generated electrons are transported via NADP+, phenazine methosulphate and menadione to nitro-BT, which is finally precipitated as formazan. Sodium azide and amytal are included to block electron transfer to cytochromes. The method proved to be of value for the qualitative demonstration of pyruvate kinase activity in tissue sections of kidneys, heart muscle and skeletal muscle. For quantitative studies and for investigating the activity of this enzyme in liver sections, the method cannot be recommended.

Enzymes of serine metabolism in normal, developing and neoplastic rat tissues

The cellular pattern of serine metabolism was conceptualized into four main areas of metabolic sequences: the biosynthesis of serine from intermediates of the glycolytic pathway (the so-called "phosphorylated pathway"); and alternative pathways of serine utilization initiated by serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. The known regulatory and adaptive properties of the enzymes involved in these pathways were reviewed in detail and key enzymes associated with each pathway (phosphoserine aminotransferase, serine dehydratase, serine aminotransferase, and serine hydroxymethyltransferase, respectively) were selected for further investigation. Tissue distribution studies in the rat revealed that whereas serine dehydratase and serine aminotransferase activities were largely confined to the liver, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were more broadly distributed. In particular in tissues with a high rate of cell turnover, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were coordinately increased. An increase in serine hydroxymethyltransferase activity coincided temporally with the incorporation of [3-14C]serine and thymidine into DNA in normal human lymphocytes during proliferation after mitogenic stimulation by phytohemagglutinin. The evidence suggested a primarily gluconeogenic role for serine dehydratase and serine aminotransferase. Serine hydroxymethyltransferase has a role in providing glycine and one-carbon folate co-factors as precursors for nucleotide biosynthesis and in some situations serves to metabolically couple the pathway of serine biosynthesis to utilization for de novo purine and pyrimidine synthesis. Multiple enzymic forms were distinguished for serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. For serine dehydratase the two cytosolic multiple forms had no apparent functional significance; the multiple forms were catalytically unmodified by conditions promoting phosphorylation-dephosphorylation in vitro. The mitochondrial form of serine aminotransferase showed adaptive responses in gluconeogenic situations, and the hypothesis was proposed that the mitochondrial isoenzyme of serine hydroxymethyltransferase is associated together with serine aminotransferase in a pathway for gluconeogenesis from protein-derived amino acids such as glycine and hydroxyproline. The adaptive behaviour of the enzymes during the neonatal development of rat liver revealed that serine aminotransferase reached a peak in the mid-suckling period at a time when gluconeogenesis is known to be increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative studies on threonine and serine dehydratases in rat liver

1. Two isoenzymes of serine (threonine) dehydratase can be separated by DEAE cellulose chromatography. 2. Each of these isoenzymes possesses different S:T (serine dehydratase activity: threonine dehydratase activity) ratio and kinetic properties. 3. In early postnatal development, the S:T ratio shows marked variation. 4. The hepatic S:T ratio also changes with dietary manipulations. Furthermore L-alanine inhibits enzymic activities towards serine and threonine to different extent. 5. The results are consistent with the hypothesis postulating the presence of two distinct proteins each having a different activity ratio towards serine and threonine.

Serine biosynthesis in human hair follicles by the phosphorylated pathway: follicular 3-phosphoglycerate dehydrogenase

The phosphorylated pathway of serine biosynthesis was demonstrated in human hair bulbs and sheaths by the formation of phosphoserine and serine from (14C)3-phosphoglyceric acid. The initial and rate limiting enzyme of the pathway, 3-phosphoglycerate dehydrogenase (3-PGDH) was demonstrated by enzyme determinations in human and rat hair follicles, human epidermis, and chicken epidermis. Follicular 3-PGDH was characterized using a sensitive fluorometric assay with NADH as a co-substrate. Monovalent cations (Na+, K+, Li+, or NH4+) were necessary for full enzyme activity. p-Hydroxymercuribenzoate inhibited activity, and activity was 3 times higher with NADH as a co-substrate than with NADPH. The apparent Km for the substrate hydroxyphosphopyruvic acid was 32.8 muM, and the apparent Km for NADH 4.8 muM similar to the Kms for other mammalian 3-PGDHs. Enzyme activity was not altered by parenteral corticosteroids, a high carbohydrate diet, low protein diet, or starvation. Enzyme activity decreased over the first 12 days of life in newborn rats. The phosphorylated pathway of serine synthesis provides a potential nondietary and nonhepatic source of serine, glycine, and their products in keratinizing tissues.