M isozyme

Figure 10.27. Isozymes of Lactate Dehydrogenase. (A) The rat heart LDH isozyme profile changes in the course of development. The H isozyme is represented by squares and the M isozyme by circles. The negative and positive numbers denote the days before and after birth, respectively. (B) LDH isozyme content varies by tissue. [(A) After W.-H. Li, Molecular Evolution (Sinauer, 1997), p. 283 (B) After K. Urich, Comparative Animal Biochemistry (S nngQrVQrlag, 1990), p. 542.]...

It has been proposed that the molecular basis for the evolution of H and M isozymes is that the M form is less susceptible to inhibition by pyruvate (36). This has been questioned since pyruvate levels in anaerobic muscle at 37° do not approach those required for inhibition (Table XXIII). Lactate levels do increase dramatically, and resistance to lactate inhibition has been suggested as an alternative explanation (267). 

Contrast the properties and roles of the H and M isozymes of lactate dehydrogenase. 

Diet group Total L isozyme (nmole/ min/g wet wt) Total M isozyme (nmole/ min/g wet wt) % L... 

Harpster, M.H. Taylor, W.C. (1986). Maize phosphoenolpyruvate carboxylases. Cloning and characterization of mRNAs encoding isozymic forms. Journal of Biological Chemistry, 261, 6132-6. 

Schiller, G., Conkle, M. T. and Grundwald, C. 1986. Local differentiation among Mediterranean populations of Aleppo pine in their isozymes. Silvae Genet. 35 11-19. 

Phosphofructokinase (PFK) is a key regulatory enzyme of glycolysis that catalyzes the conversion of fructose-6-phosphate to fructose-1,6-diphosphate. The active PFK enzyme is a homo- or heterotetrameric enzyme with a molecular weight of 340,000. Three types of subunits, muscle type (M), liver type (L), and fibroblast (F) or platelet (P) type, exist in human tissues. Human muscle and liver PFKs consist of homotetramers (M4 and L4), whereas red blood cell PFK consists of five tetramers (M4, M3L, M2L2, ML3, and L4). Each isoform is unique with respect to affinity for the substrate fructose-6-phosphate and ATP and modulation by effectors such as citrate, ATP, cAMP, and fructose-2,6-diphosphate. M-type PFK has greater affinity for fructose-6-phosphate than the other isozymes. AMP and fructose-2,6-diphosphate facilitate fructose-6-phosphate binding mainly of L-type PFK, whereas P-type PFK has intermediate properties. 

Pyruvate kinase (PK) is one of the three postulated rate-controlling enzymes of glycolysis. The high-energy phosphate of phosphoenolpyruvate is transferred to ADP by this enzyme, which requires for its activity both monovalent and divalent cations. Enolpyruvate formed in this reaction is converted spontaneously to the keto form of pyruvate with the synthesis of one ATP molecule. PK has four isozymes in mammals M, M2, L, and R. The M2 type, which is considered to be the prototype, is the only form detected in early fetal tissues and is expressed in many adult tissues. This form is progressively replaced by the M( type in the skeletal muscle, heart, and brain by the L type in the liver and by the R type in red blood cells during development or differentiation (M26). The M, and M2 isozymes display Michaelis-Menten kinetics with respect to phosphoenolpyruvate. The Mj isozyme is not affected by fructose-1,6-diphosphate (F-1,6-DP) and the M2 is al-losterically activated by this compound. Type L and R exhibit cooperatively in... 

N10. Noguchi, T., Inoue, H., and Tanaka, T., The M,- and M2-type isozymes of rat pyruvate kinase are produced from the same gene by alternative RNA splicing. J. Biol. Chem. 261,13807-13812 (1986). 

Sakakibara, M., Mukai, T., Yatsuki, H and Hori, K Human aldolase isozyme gene The structure of multispecies aldolase B mRNAs. Nucleic Acids Res. 13,5055-5069 (1985). 

Ul. Uenaka, R., Nakajima, H., Noguchi, T., Imamura, K Hamagauchi, T Tomita, K Yamada, K., Kuwajima, M Kono, N Tanaka, T and Matsuzawa, Y., Compound heterozygous mutations affecting both hepatic and erythrocyte isozymes of pyruvate kinase. Biochem. Biophys. Res. Commun. 208,991-998 (1995). 

Owing to the fact that ethyl ethers are especially effective substrates for CYP1A1 [184], the probe possesses an ethyl group on the phenolic oxygen of the trimethyl lock. In vitro, fluorescence was manifested by CYP1A1 isozyme with Kcat/KM 8.8 x 103 M-1s 1 and KM 0.09 pM. In cellulo, the probe revealed the induction of cytochrome P450 activity by the carcinogen 2,3,7,8-tetrachlorodi-benzo-p-dioxin (TCDD), and its repression by the chemoprotectant resveratrol. 

Smith M, Hopkinson DA, Harris H. Alcohol dehydrogenase isozymes in adult human stomach and liver evidence for activity of the ADH3 locus. Ann Hum Genet (London) 1972 35 243-253. 

Avidor-Reiss T, Nevo I, Saja D, Bayewitch M, Vogel Z. Opiate-induced adenylyl cyclase superactivation is isozyme-specific. J Biol Chem 1997 272 5040-5047. 

Because many other chemicals can affect the enzymes responsible for n-hexane metabolism (see Section 2.3.3, Metabolism), the possibility of interactions is a significant concern. The initial step in n-hexane metabolism is oxidation to a hexanol by a cytochrome P-450 isozyme other chemicals can induce these enzymes, possibly increasing the rate of metabolism to the neurotoxic 2,5-hexanedione, or competing with M-hexanc and its metabolites at enzyme active sites, reducing the rate of metabolism. Interactive effects can be concentration and/or duration dependent. 

There is no experimental evidence adequate to evaluate whether metabolism of M-hcxanc is different in children. Similarly, there is no information available from animal experiments. The initial step in -hexane metabolism in animals is a hydroxylation step catalyzed by a P-450 enzyme. Since some of these enzymes are developmentally regulated, it would be of interest to know (1) if there are specific P-450 isozymes involved in -hexane hydroxylation and, (2) if so, are these isozymes known to be developmentally regulated ... 

Toftgard R, Haaparanta T, Eng L, etal. 1986. Rat lung and liver microsomal cytochrome P-450 isozymes involved in the hydroxylation of M-hexane. Biochem Pharmacol 35(21) 3733-3738. 

Grant DM, Blum M, Beer M, et al. Monomorphic and polymorphic human arylamine N-acetyltransferases a comparison of liver isozymes and expressed products of two cloned genes. Mol Pharmacol 1991 39(2) 184-191. 

Steinke M, Wolfe GV, Kirst GO (1998) Partial characterisation of dimethylsulfoniopropionate (DMSP) lyase isozymes in 6 strains of Emiliania huxleyi. Mar Ecol Prog Ser 175 215-225... 

Lewandowski M, Levi P, Hodgson E. 1989. Induction of cytochrome P-450 isozymes by mirex and chlordecone. J Biochem Toxicol 4(3) 195-199. 

M. Hosokawa, T. Maki, T. Satoh, Multiplicity and Regulation of Hepatic Microsomal Carboxylesterases in Rats , Mol. Pharmacol. 1987, 31, 579-584 M. Hosokawa, T. Satoh, Effects of Hypophysectomy and Pituitary Hormones on Hepatic Microsomal Carboxylesterase Isozymes in Male Rats , Res. Commun. Chem. Pathol. Pharmacol. 

Haas, A. L., Bright, P. M., and Jackson, V. E. Functional diversity among putative E2 isozymes in the mechanism of ubiquitin-histone ligation./. Biol. Chem. 1988, 363, 13268-75. 

Coutts RT, Su P, Baker GB, Daneshtalab M. 1993. Metabolism of imipramine in vitro by isozyme CYP2D6 expressed in a human cell line, and observations on metabolite stability. J Chromatogr B Biomed Appl 615 265. 

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