Liver multiple molecular forms

The liver enzyme exists in two forms, E and S, which differ only by some six residues in their amino acid sequence.13 Only the S is active toward 3-/3-hydroxysteroids, but both forms are active toward ethanol. None of the known amino acid differences is located in the subunit interfaces. Accordingly, E and S chains combine in statistical ratios to form EE, SS, and ES dimers. These different species are termed isozymes, which means that they are multiple molecular forms of the same enzyme. When it is isolated from liver, the enzyme consists of about 40 to 60% of the EE dimer, the remainder being SS and ES. 

The purification of acid phosphatase from the human liver and the description of its properties do not appear to have been accomplished. Partly, this may be due to the inherent difiBculty of obtaining normal, fresh human material in amounts substantial enough for purification. However, because of the cellular and physiological importance of acid phosphatase, it is advisable to describe in the present section the purifications of the enzyme from rat and bovine liver. Moreover, since these purifications were accomplished with the awareness that acid phosphatase from this source might be present in multiple molecular forms, the descriptions will naturally involve a consideration of the isoenzymes and their properties. 

This order of evaluation (biochemical, first starch gel, second) is amply justified from our own experiences and the consensus of opinion at a recent symposium on multiple molecular forms of enzymes. In particular, heterogeneity of human intestinal alkaline phosphatase on starch gel was reported by Moss (M34) and Fishman and Kreisher (F9, K25). A certain amount of LPSAP intestinal enzyme may occupy the same position that liver alkaline phosphatase migrates to. On the other hand, more than one band in nonliver positions can be produced by intestine. Another similar situation is seen with placental alkaline phosphatase, which may show three bands. Consequently, the starch-gel data can be correlated with biochemical studies only if the nature and organ source of the preparation are known in advance. 

In rat liver ADH the presence of multiple molecular forms has been correlated to disulfide bridges involving the ligands to this zinc atom (Section II,B,3). These forms are active in ethanol oxidation (62). The lobe region which binds zinc is thus, in all probability, not essential for the catalytic action of alcohol oxidation. It has been suggested (133,134) that the extra zinc atom is essential for the structural stability of the enzyme. There is no evidence in the structure that this lobe region is necessary either for tertiary or quaternary structure stabilization. From the structural point of view, this region looks much more like a second catalytic center. The zinc atom is situated in one side of an obvious cleft into which the lone pair electrons of the sulfur atom of Cys-97 project. 

There are several reports by Jones and co-workers dealing with the purification, properties and conformation of the orotate phosphoribosyltransferase and orotidylic acid decarboxylase enzyme complex present in mouse Ehrlich ascites cells [135-137]. Multiple molecular forms of orotidylic acid decarboxylase from human erythrocytes and human liver were studied by O Sullivan and co-workers [138,139]. A bifunctional enzyme complex of orotate phosphoribosyltransferase and orotidylic acid decarboxylase occurs also in mouse liver and brain [140], regardless of the developmental stage of the animal. Both enzyme activities remained co-ordinate in fetal, neonatal, immature and adult liver and brain. 

C6 Campbell, M. T., Gallagher, N. D. and O Sullivan, W. J. Multiple molecular forms of orotidylate decarboxylase from human liver. Biochem. Med., 17, 128-140 (1977)... 

GonzMes, C., Ureta, T., Sanchez, R., Niemeyer, H. Multiple molecular forms of ATP Hexose 6-phosphotransferase from rat liver. Biol. Biochem. Res. Commun. 16, 347-357(1964)... 

O-Methylation. Catechol O-mcthy I transferase occurs in the soluble fraction of several tissues and has been purified from rat liver. The purified form has a molecular weight 23,000 daltons, requires S-adenosyIrnclhioninc and Mg+, and catalyzes the methylation of epinephrine, norepinephrine, and other catechol derivatives. There is evidence that this enzyme exists in multiple forms. 

Horse liver alcohol dehydrogenase was first crystallized by Bonnichsen and Wasson in 1948 (34) An acidic minor component was isolated by Dalziel (35), and different forms were later shown to exist (36,37). Neither of these studies revealed the true isozyme pattern of horse liver alcohol dehydrogenase, and an increasing number of different molecular forms have since been characterized. The multiplicity is a result of the synthesis of different types of subunits as well as of the occurrence of secondary modified forms. 

Multiple forms of GST have been demonstrated in the liver of many mammalian species multiple forms also occur in insects. Most GSTs are soluble dimeric proteins with molecular weights ranging between 45,000 and 50,000 daltons. All forms appear to be nonspecific with respect to the reaction types described, although the kinetic constants for particular substrates vary from one form to another. They are usually named from their chromatographic behavior. At least two are membrane-bound glutathione transferases, one of which is involved in metabolism of xenobiotics and is designated... 

One of the most interesting problems concerning the cytochrome P-450 system has been its multiplicity, for it is difficult to conceive of a single enzyme system having the ability to oxidise in a specific way such a wide variety of substrates. Current data suggest that there are at least three forms of the enzymes present in rat liver microsomes. Optical spectra reveal two forms of the CO-bound reduced cytochrome P-450, one with a maximum at 448 nm (such as is induced by 3-methylcholanthrene) and one at 450 nm (as is induced by phenobarbital), and SDS (sodium dodecyl sulphate) polyacrylamide gel electrophoresis shows three peaks corresponding to molecular weights of 44000 (phenobarbital), 50000, and 53000 (3-methylcholanthrene) [117]. Differences are also seen in the ESR spectra of P-450 in rat liver microsomes induced by different chemicals and there are further differences when... 

GGT activity in serum comes primarily from liver. The enzyme in serum is heterogeneous with respect to both net molecular charge (e.g., shown by electrophoresis) and size. These forms appear to derive from posttranslational modifications of a single type of enzyme molecule rather than to be due to the existence of true isoenzymes. For example, high molecular weight forms may represent the release of cell membrane fragments into the circulation. Despite numerous investigations, clear correlations between patterns of multiple forms and particular diseases cannot be discerned. 

The discovery of c5 ochromes P450 in mammalian tissues rich with these proteins, such as liver and the adrenal gland, resulted in intense scrutiny of their roles in xenobiotic metabolism and endogenous functions. Following their discovery came the realization that mammalian proteins required electron donor systems for activity, either NADPH-cytochrome P450 reductase (CPR) or adrenodoxin and adrenodoxin reductase in the endoplasmic reticulum or the mitochondria respectively Protein biochemistry and molecular biology revealed the multiplicity of CYP forms and mammalian genomes exhibited CYP diversity. 

The molecular weight of cathepsin B from various organs and tissues is in the range of 24,000-28,000. The highest values in this range have been determined by gel filtration methods. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified enzyme is often nicked between 23,000 and 5000 and showed multiple forms on electrofocusing (p/ 4.5-5.6). The N-terminal amino acid of rat liver cathepsin B is leucine. It contains carbohydrate 19) but the exact sugar content has not yet been measured. 

Acid Phosphatases.—The molecular properties of the multiple forms of an acid phosphatase from horse liver have been studied. ... 

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