Cytoplasmic enzyme

Several enzymes, none of which are completely specific for the enkephalins, are known to cleave Leu- and Met-enkephalin at various peptide bonds. The main enzymes that degrade enkephalin are 2inc metaHopeptidases. The first enkephalin-degrading enzyme to be identified, an aminopeptidase which cleaves the amino terminal Tyr-Gly bond (179), has been shown to be aminopeptidase-N (APN) (180). It is a cytoplasmic enzyme which is uniformly distributed throughout the brain. The increased analgesic activity of synthetic enkephalins substituted by D-amino acids at position 2, eg,... 

Alcohol dehydrogenase is a cytoplasmic enzyme mainly found in the liver, but also in the stomach. The enzyme accomplishes the first step of ethanol metabolism, oxidation to acetaldehyde, which is further metabolized by aldehyde dehydrogenase. Quantitatively, the oxidation of ethanol is more or less independent of the blood concentration and constant with time, i.e. it follows zero-order kinetics (pharmacokinetics). On average, a 70-kg person oxidizes about 10 ml of ethanol per hour. 

The reaction of choline with mitochondrial bound acetylcoenzyme A is catalysed by the cytoplasmic enzyme choline acetyltransferase (ChAT) (see Fig. 6.1). ChAT itelf is synthesised in the rough endoplasmic reticulum of the cell body and transported to the axon terminal. Although the precise location of the synthesis of ACh is uncertain most of that formed is stored in vesicles. It appears that while ChAT is not saturated with either acetyl-CoA or choline its synthesising activity is limited by the actual availability of choline, i.e. its uptake into the nerve terminal. No inhibitors of ChAT itself have been developed but the rate of synthesis of ACh can, however, be inhibited by drugs like hemicholinium or triethylcholine, which compete for choline uptake into the nerve. 

Tyrosine is converted to dopa by the cytoplasmic enzyme tyrosine hydroxylase. This is the rate-limiting step 5 x 10 M) in DA synthesis, it requires molecular O2 and Fe + as well as tetrahydropterine (BH-4) cofactor and is substrate-specific. It can be inhibited by a-methyl-p-tyrosine, which depletes the brain of both DA and NA and it is particularly important for the maintenance of DA synthesis. Since the levels of tyrosine are above the for tyrosine hydroxylase the enzyme is normally saturated and so it is not possible to increase DA levels by giving tyrosine. 

Figure 9.4 The synthesis and metabolism of 5-HT. The primary substrate for the pathway is the essential amino acid, tryptophan and its hydroxylation to 5-hydrox5dryptophan is the rate-limiting step in the synthesis of 5-HT. The cytoplasmic enzyme, monoamine oxidase (MAOa), is ultimately responsible for the catabolism of 5-HT to 5-hydroxyindoleacetic acid...

XOR is a cytoplasmic enzyme and a ready source of electrons for transfer to molecular oxygen to form reactive oxygen species such as superoxide and peroxide. It is therefore thought to be involved in free radical-generated tissue injury and has been implicated in the pathogenesis of ischemia-reperfusion damage. Moreover, it has recently been implicated in the production of peroxynitrite (89), and carbonate radical anion (92), both potent biological oxidants. Its exact role in lipid peroxidation, inflammation, and infection needs... 

This class of receptors transmits signals carried by hormones and growth factors. The structure consists of an extracellular domain for binding ligands and a cytoplasmic enzyme domain. The function of kinases is to enable phosphorylation. Phosphorylation regulates most aspects of cell life. 

The Other enzyme in catecholamine catabolism is catecholamine 0-methyltransferase (COMT), a cytoplasmic enzyme that uses S-adenosyl-methionine to methylate the 3-OH of catecholamines and render them inactive. The methylated compounds are not taken up into the synapse. 

Additional information <14> (<14> transgenic mice lacking mitochondrial enzyme or both mitochondrial and cytoplasmic enzyme [63]) [63]... 

Figure 17.13 Barriers separating periplasmic enzymes (gram-negative bacteria) and cytoplasmic enzymes (gram-negative and grampositive bacteria) from substrates in the surrounding environment (Madigan et al., 2000).

However, little is currently known about its synthesis. The protein component may be assembled on the ribosomes by the normal mechanism of protein synthesis.324 L-Proline is known to be the precursor of the hydroxy-L-proline found in the glycoprotein,324,325 hydroxylation of the peptide-bound L-proline being catalyzed, in carrot cells, by cytoplasmic enzymes.324... 

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