Formate activating enzyme

Grignani et al. (G14) studied several of the enzymes of folate metabolism in human epidermis—both normal and psoriatic. Increased levels of folate reductase were found in the psoriatic lesion, and further enzyme could be induced by treatment of the patients with amethopterin. By contrast, formate-activating enzyme, 5,10-methylenetetrahydrofolate dehydrogenase, serine hydroxylase, and cyclohydrolase were normal in the psoriatic lesion. Formiminotetrahydrofolate transferase could not be measured either in normal or psoriatic skin. The activities of the above enzymes as well as the absence of the transferase are similar to the findings for small bowel but not to other tissues studied. How these findings... 

If different patients with acute leukemia were studied, the amount of 14C-formate incorporation was correlated to the activity of the formate activating enzyme as illustrated in figure 3. In addition, it was found that the rate of 14C-formate incorporation into purine nucleotides depends on the concentration of tetrahydrofolate (FH ) in the incubation mixture (fig. 4). 

Components I Components of the formate activating enzyme II In addition KHCO, glutamine glycine,ri-bose-5-phosphate... 

Activities of the formate activating enzyme and incorporation rates of 14C-formate into purine nucleotides in cell-free extracts of normal leukocytes and bone marrow cells and of leukemic cells. 

Activity of the formate activating enzyme in leukemic blast cells of 5 patients with acute leukemia under the action of 6-MP and allopurinol... 

UNGER,K.W., and R SILBER Studies on the formate activating enzyme Kinetics of 6-Mercaptopurine inhibition and stabilisation of the enzyme.Biochim.biophys.Acta (Amst,) 89, 167 (1964). 

Usually, a rapid binding step of the inhibitor I to the enzyme E leads to the formation of the initial noncovalent enzyme-inhibitor complex E-I. This is usually followed by a rate determining catalytic step, leading to the formation of a highly reactive species [E—I ]. This species can either undergo reaction with an active site amino acid residue of the enzyme to form the covalent enzyme-inhibitor adduct E—I", or be released into the medium to form product P and free active enzyme E. 

Many enzymes (see Chapters 14 to 16) derive at least some of their catalytic power from oligomeric associations of monomer subunits. This can happen in several ways. The monomer may not constitute a complete enzyme active site. Formation of the oligomer may bring ail the necessary catalytic groups together to form an active enzyme. For example, the active sites of bacterial glutamine synthetase are formed from pairs of adjacent subunits. The dissociated monomers are inactive. 

The catalytically active enzyme substrate complex is an interactive structure in which the enzyme causes the substrate to adopt a form that mimics the transition-state intermediate of the reaction. Thus, a poor substrate would be one that was less effective in directing the formation of an optimally active enzyme transition-state intermediate conformation. This active conformation of the enzyme molecule is thought to be relatively unstable in the absence of substrate, and free enzyme thus reverts to a conformationally different state. 

Most enzymes consist of several identical or different subunits. It is known that subunits of similar activity but different origin, which therefore differ in size and amino acid composition and sequence, replace each other in oligomeric enzymes, leading to the formation of enzyme chimeras of catalytic activity 47). The feasibility... 

Stimulation and inhibition of the enzyme by the GPCR-G-protein cycle occur by analogous mechanisms. Agonists induce hormone receptors to increase a Ga-GDP-GTP exchange and subsequent Ga 3y dissociation (GDP-a py + GTP GTP-ax + [3y + GDP) (Fig. 4). Consequently, agents that affect either the dissociation of either G or Gs, or the association of their respective as, a , or (3y subunits with adenylyl cyclase could affect rates of cAMP formation in enzyme preparations or in intact cells and tissues. There are several important examples. Gas is stably activated by poorly hydrolyzable analogs of GTP, e.g. GTPyS... 

Fibrinolytics. Figure 3 Plasminogen activation (a) Kinetics of plasminogen activation by uPA (urokinase-type) and tPA (tissue-type) plasminogen activators. Effect of fibrin (b) Ternary complex formation between enzyme (tPA), substrate (Pg) and cofactor (F) Abbreviations plasmin (P), fibrin (F), plasminogen (Pg). Plasmin, formed in time, is expressed in arbitrary units. 

Imhoff D, JR Andreesen (1979) Nicotinic acid hydroxylase from Clostridium barkeri selenium-dependent formation of active enzyme. FEMS Microbiol Lett 5 155-158. 

Lequea et al. used the activity of tyrosine apodecarboxylase to determine the concentration of the enzyme cofactor pyridoxal 5 -phosphate (vitamin B6). The inactive apoenzyme is converted to the active enzyme by pyridoxal 5 -phosphate. By keeping the cofactor the limiting reagent in the reaction by adding excess apoenzyme and substrate, the enzyme activity is a direct measure of cofactor concentration. The enzymatic reaction was followed by detecting tyramine formation by LCEC. The authors used this method to determine vitamin B6 concentrations in plasma samples. 

The third format is the Hithunter assay and is based on the ability of two enzyme fragments to recombine into active enzyme. The principle behind the assay is shown in Fig. 7. 

Figure 20.4 Reduction of the disulfide bonds within the hinge region of an IgG molecule produces half-anti-body molecules containing thiol groups. Reaction of these reduced antibodies with a maleimide-activated enzyme creates a conjugate through thioether bond formation.

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