Brain enzymes

Microwaves are also used for the rapid inactivation of brain enzymes in rodents (160). Microwave power at high levels of kilowatts is appHed by means of a waveguide appHcator to achieve a rapid sacrifice of the rodent. 

Ueda N, Kurahashi Y, Yamamoto S, Tokunaga T. Partial purification and characterization of the porcine brain enzyme hydrolyzing and synthesizing anandamide. J Biol Chem 1995 270 23823-23827. 

The CNS contains much smaller amounts of drug-metabolizing enzymes than does the liver. The concentrations of the main enzymes in the brain, members of the cytochrome P450 (CYP) superfamily, are only 0.25% of concentration in the liver. But the brain enzymes are not uniformly distributed, as they are in the liver they are concentrated in specific brain areas. Theoretical models have explained that drug metabolism in the CNS cannot influence drug distribution in the blood, but there are marked differences in brain tissue levels depending on the presence... 

One result of the selective localization of brain enzymes is that astrocytes must provide certain substrates (e.g. glutamine) to neurons for replenishment of the neuronal TCA cycle and for neurotransmitter synthesis [58]. Thus, astrocytes and neurons are essential partners in brain function. See also discussion in Chapter 15. 

Sastry, K.V. and K. Sharma. 1980. Diazinon effect on the activities of brain enzymes from Ophiocephalus (Channa) punctatus. Bull. Environ. Contam. Toxicol. 24 326-332. 

The roles of Na(I), K(I), and Ca(II) in neurochemistry are well known, but it is also apparent that Fe and Cu enzymes can control neurotransmitter biosynthetic pathways, and there are millimolar levels of Zn2+ in the hippocampus during neurotransmission (487, 488). Moreover, Mn is abundant in brain enzymes such glutamine synthase and superoxide dismutase. 

This pyridoxal-phosphate-dependent enzyme [EC 4.1.1.15] catalyzes the conversion of L-glutamate to 4-aminobutanoate and carbon dioxide. The mammalian brain enzyme also acts on L-cysteate, 3-sulfino-L-alanine, and L-aspartate. 

Haldane is also valid for the ordered Bi Bi Theorell-Chance mechanism and the rapid equilibrium random Bi Bi mechanism. The reverse reaction of the yeast enzyme is easily studied an observation not true for the brain enzyme, even though both enzymes catalyze the exact same reaction. A crucial difference between the two enzymes is the dissociation constant (i iq) for Q (in this case, glucose 6-phosphate). For the yeast enzyme, this value is about 5 mM whereas for the brain enzyme the value is 1 tM. Hence, in order for Keq to remain constant (and assuming Kp, and are all approximately the same for both enzymes) the Hmax,f/f max,r ratio for the brain enzyme must be considerably larger than the corresponding ratio for the yeast enzyme. In fact, the differences between the two ratios is more than a thousandfold. Hence, the Haldane relationship helps to explain how one enzyme appears to be more kmeticaUy reversible than another catalyzing the same reaction. 

While this model explained the action of the brain enzyme on a number of hexose substrates and nonsubstrate inhibitory analogs, the mode had its weaknesses. It assumed that the other conformations of a hexose that are in equilibrium with the active conformer act as competitive inhibitors relative to this conformer. One cannot evaluate the effect of a competitive inhibitor which is present in a constant proportion relative to the active substrate by initial velocity measurements. Moreover, the use of apparent Michaelis constants may not provide accurate estimates of affinity, which is more directly related to a dissociation constant. The chief limitation of the model, however, is that an equally great number of experimental facts can be satisfactorily explained in terms of a simpler scheme involving the binding and phosphorylation of the Cl conformer. Furthermore, one can understand more directly how the enzyme can phosphorylate glucopyranose and fructofuranose equally well. 

MAOIs work by inhibiting a brain enzyme called monoamine oxidase. Patients taking MAOIs must avoid a substantial list of foods that can interact with the medication and cause dangerously high blood pressure. 

C, partially purified brain enzyme, very stable [2]... 

Whereas the rabbit muscle (68) and brain preparations (129) required 1-10 mM mercaptoethanol and KC1 or LiCl for stability over extended time periods, the preparation described by Lee was not affected by reducing or oxidizing agents (130). Multivalent anions, such as tripolyphosphate, 3-iso-AMP, ATP, and GTP, but not substrate, stabilized the calf brain enzyme against heat inactivation (129, 131). 

In general, AMP aminohydrolase specificities have not been thoroughly defined perhaps because of difficulties until recently in obtaining pure enzyme. In addition to AMP and dAMP, the muscle enzyme catalyzes the deamination of V -methyl AMP, iV -ethyl AMP, for-mycin-5 -monophosphate, adenosine-5 -monosulfate, adenosine-5 -phos-phoramidate, adenosine, ADP (133), adenosine-5 -phosphorothioate, and 6-chloropurine 5 -ribonucleotide (124) ATP, GMP, CMP, 2 -AMP, 3 -AMP, 3, 5 -cyclic AMP, 3-iso-AMP, V-methyl AMP, toyocamycin-5 -monophosphate, tubercidin-5 -monophosphate, and 6-mercaptopurine-5 -ribonucleotide are not deaminated (133). The elasmobranch fish muscle, carp muscle, and avian brain enzymes appear to be specific for AMP and dAMP (123, 125, 126). Extracts from pea seed and erythrocytes and the purified calf brain enzyme are specific for AMP (48, 131, 134). 

The kinetic parameters of various muscle AMP aminohydrolases presented in Table V (51, 68, 122, 124-127) are similar except for the lower specific activities exhibited by the fish enzymes for which no criteria of homogeneity are presently available. Specific activities reported for brain enzymes not shown in Table V are 15 /amoles/min/mg for calf (129) and 30 /amoles/min/mg for chicken (123). Although the pH optimum for AMP deamination varies depending upon the source, it normally occurs in a range from pH 5.9 to 7.1 (48, 125, 126, 129, 130, 135-137). 

In the absence of activators AMP aminohydrolase from brain (149), erythrocytes (143, 150), muscle (145), and liver (128) gave sigmoid curves for velocity vs. AMP concentration which were hyperbolic after the addition of monovalent cations, adenine nucleotides, or a combination of monovalent cations and adenine nucleotides. For the rabbit muscle enzyme (145), addition of K+, ADP, or ATP produced normal hyperbolic saturation curves for AMP as represented by a change in the Hill slope nH from 2.2 to 1.1 Fmax remained the same. The soluble erythrocyte enzyme and the calf brain enzyme required the presence of both monovalent cations and ATP before saturation curves became hyperbolic. In contrast, the bound human erythrocyte membrane enzyme did not exhibit sigmoid saturation curves and K+ activation was not affected by ATP (142). 

Partially purified preparations of guanine aminohydrolase (EC 3.5.4.3) have been reported from rabbit liver (68,186), rat liver (61), Clostridium addwrici (187), rat brain (60, 188, 189), and lingcod muscle (190). The rat brain enzyme (60) occurs in both the mitochondrial and supernatant fraction the latter fraction yielded two forms, A and B from DEAE-cellulose, which were subsequently purified 70- and 600-fold, respectively. Form B had a specific activity of 290 /unoles/min/mg. Kinetic, immunochemical, and electrophoretic studies revealed that the mitochondrial enzyme was distinct from supernatant enzyme B. A distinction between the supernatant A and B forms was less certain (60). 

The enzyme has been partially purified (70-fold) from 38,000 X 9 supernatant fluid from sheep brain homogenates by Ipata (55-58). Thq enzyme (MW 140,000) is reported to be specific for 5 -AMP and 5 -IMP although the substrate specificity does not appear to have been examined closely. 2 - and 3 -AMP are not hydrolyzed (56). Unlike the enzyme from many sources the brain enzyme does not require divalent cations and indeed Co2+, which stimulates several other 5 -nucleotidases, was inhibitory at 5 mM. The enzyme is strongly inhibited by very low concentrations of ATP, UTP, and CTP (50% inhibition by 0.3 pM ATP) but not by GTP. 2 -AMP, 3 -AMP, and a variety of other nucleoside monophosphates, nucleosides, and sugar phosphates do not inhibit. A kinetic examination of ATP, UTP, and CTP inhibition (56-58) revealed that inhibition curves were sigmoidal, indicating cooperativity between inhibitor molecules and an allosteric type of interaction between inhibitor and protein. The metabolic significance of ATP inhibition is... 

The most notable similarities relate to activation and inactivation by metal ions and other materials. In most instances (Table I) each is activated by one or more of the cations Co2+, Mn2+, Ni2+, Mg2+, or Ca2+ of which the former three are usually most effective. Here again, however, there are differences. Thus, the B. atrox enzyme is not activated by ions, but they serve to reverse EDTA inhibition (23). The rat liver lysosomal enzyme is also not activated by divalent metals (SI) the sheep brain enzyme does not seem to require divalent cation and in fact it is inhibited by Co2+ (57). 

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