Magnesium ions liver

Niwaguchi et al. (14-16) reported that when rat liver homogenates were incubated in a system containing 14C-L-glutamate, glucose, DPN, adenosine triphosphate (ATP), magnesium ions, cytochrome c, and fu-marate, an acidic compound was formed which could be identified as pyrrolidone carboxylic acid by infrared spectroscopy, electrophoresis,... 

CPSI catalyzes the formation of carbamoyl phosphate from bicarbonate, ammonium, and two adenosine triphosphate molecules (Fig. 18-1).This first step of the urea cycle occurs in the mitochondrial matrix and assimilates the first of the two nitrogen atoms that will eventually be found in urea. While two ATP molecules are hydrolyzed, there is formation of a lower energy bond in carbamoyl phosphate. CPSI is a homodimer that accounts for 15-30% of the total protein mass in liver mitochondria. jV-Acetylglutamate (NAG) is an essential allosteric activator of CPSI activity, and magnesium ions are also required for its activity. 

Pyruvate carboxylase is a well-known Mn metalloenzyme. The enzyme is a tetramer and contains one biotin cofactor per subunit and one divalent cation per subunit. The enzyme from calf liver, for example, contains four tightly boimd Mn atoms. The enzyme from chicken liver contains two Mn atoms and two Mg atoms. Raising chickens on an Mn-deficient diet results in the production of an Mn-free enzyme, where magnesium ions replace the usually occurring manganese ions. The Mg-containing enzyme is catalytically active, leaving the requirement of the enzyme for Mn in question (Scrutton et al, 1972). 

D-Glucosamine 6-phosphate is hydrolyzed by the D-glucose-6-phosphatase of rat-liver mitochondria. The rate of this hydrolysis is about 8 % of that of D-glucose 6-phosphate hydrolysis. A phosphatase which preferentially catalyzes the hydrolysis of D-glucosamine 6-phosphate has been prepared from Neurospora crassa. This enzyme is not stimulated by magnesium ions and has an optimum activity between pH 6 and 7.5. It appears to be distinct from acid, alkaline, and other specific phosphatases. 

Henderson, A. R., The effect of feeding with a tryptophan-free amino acid mixture on rat liver magnesium ion-activated deoxyribonucleic acid-dependent ribonucleic acid polymerase, Biochem. ]., 120[1], 205, 1970. 

Figure 12 Structure of bovine liver lipoyltransferase modified from PDB file 2E5A. The protein is in complex with lipoyl-AMP, shown in stick format. A magnesium ion in the binding pocket is shown as a red sphere. The structure was prepared using the PyMOL Molecular Graphics (http //www.pymol.org).

An in-vitro nuclear system prepared from HeLa cells, described by Friedman and Mueller (1968), appears to continue the DNA replication process observed in vivo. The system requires intact nuclei, the four deoxyribonucleoside triphosphates, magnesium ion, ATP, and, in addition, a heat-labile cytoplasmic factor. The activity of the system was similar to the DNA synthetic activity observed in intact cells in synchronized culture (Friedman and Mueller, 1968). Cytoplasmic factors also appear to stimulate in-vitro nuclear systems prepared from normal and regenerating rat livers (De Beilis, 1969). The cytoplasmic factors are present in both normal and regenerating liver cytoplasm and stimulate nuclear DNA synthesis in both systems. The stimulation was most marked using regenerating liver factors and normal liver nuclei (De Beilis, 1969). When mouse liver nuclei are recombined with cell free cytoplasmic extracts from mouse ascitic or L-cells active in DNA synthesis there is a marked stimulation of DNA synthesis in the isolated nuclei (Thompson and McCarthy, 1968). Cytoplasmic preparations from HeLa cells also stimulated DNA synthesis in mouse liver nuclei. 

Extracts of acetone-dried pigeon liver catalyze the synthesis of GSH from glutamic acid, cysteine, and glycine provided magnesium ions and ATP are added (2). Marked stimulation of sjmthesis is provided by potassium ions. After the removal of hydrolytic enzymes from the crude extract, a net synthesis of GSH can be demonstrated by means of the glyoxalase assay (2). The close correspondence between the values obtained by the glyoxalase assay and by the isotopic technique shows that in this system the incorporation of a labeled amino acid into GSH is a measure of de novo synthesis. 

As shown in Fig. 3, they studied the formation of cholesterol ester With time in rat liver microsomes in imidazole buffer. As can be seen, in the presence of magnesium there was a considerable decline in the amount of cholesterol ester formed, but that it could be completely prevented or blocked if potassium fluoride and EDTA were included in the incubation mixture. In a sepairate experiment, they studied cholesterol ester formation as a function of magnesium ion concentration in the medium. Again, the ACAT activity declined with increasing magnesium in the medium, whereas inclusion of EDTA together with magnesium, stabilized the enzyme activity. This information is consistent with the data from our laboratory which demonstrates the presence in microsomes of a Mg++ sensitive phosphatase which inactivates 7a-hydroxylase. 

The major ions in different organs and body fluids of euryhaline fish have been studied by a number of authors. The concentrations of sodium, potassium, magnesium and chloride were usually more concentrated in fish taken from sea water than in those from fresh water, the effect being shown in blood, kidney, liver, various secondary muscles and urine. The trend was less clear in the case of swimming muscle, as were the values for calcium (reviewed by Love, 1970, Table 30). All the ions mentioned above were much more concentrated in the urine of the fish from the sea, urine being one channel by which these salts are excreted. A fish with remarkable ability to control its internal milieu is the tilapia, in which the total sodium in the body increases by only 30% when it is transferred from fresh water to doublestrength sea water (Potts et al., 1967). 

The total concentration of Mn in the liver cell is about 35 iM, about 1/1000th that of magnesium. Free Mn occurs at about 0.2 to 1.0 iM in the liver cell. Loosely bound and freely exchangeable Mn occur at about 12. iM, with the rest of the ion occurring tightly bound to proteins. Mitochondria contain a concentration of Mn equivalent to 300 iM, about 1/100th that of mitochondrial magnesium (Senior et al, 1980). 

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