Specific relative activities

Substrate specificity relative activity in parenthesis) 3.4- Dihydroxyphenylacetic acid (100) 3.4- Dehydroxyphenylpropionic acid (2.9) N-Formyl dopa (0.9) Dopa (0.3) Protocatechuic acid (0.13) Protocatechuic acid (100) 3,4-Dihydroxy-9,10-secoandrosta-1,3,5 (10)-triene-9,17-dione-3-lsopropylcatechol (100) 3-t-Butyl-5-methylcatechol (82.4) 3- Methylcatechol (7.4) 4- Methylcatechol (4.3) Catechol (0.6)... 

Most chemical iavestigations with plutonium to date have been performed with Pu, but the isotopes Pu and Pu (produced by iatensive neutron irradiation of plutonium) are more suitable for such work because of their longer half-Hves and consequendy lower specific activities. Much work on the chemical properties of americium has been carried out with Am, which is also difficult to handle because of its relatively high specific alpha radioactivity, about 7 x 10 alpha particles/(mg-min). The isotope Am has a specific alpha activity about twenty times less than Am and is thus a more attractive isotope for chemical iavestigations. Much of the earher work with curium used the isotopes and Cm, but the heavier isotopes... 

The number, relative activity, and state of occupancy of the specific receptors on the plasma membrane or in the cytoplasm or nucleus. 

Very limited information is available through direct enzyme studies for DszA. However, its specificity for couple of possible intermediates resulting from DszC reaction has been studied. The DszA enzyme was capable of converting dibenz[c,e][l,2]oxanthiin 6,6-dioxide (sultone) as well as sultine to 2,2 dihydoxybiphenyl (DHBP), in addition to its inherent activity towards DBT sulfone. However, the relative activities were lower than the activity towards DBT sulfone (54% and 23%, respectively). The proposed pathway for this reaction is given in Fig. 2 [26,153],... 

Versus the Nafion catalyst (random copolymer) higher specific catalytic activity and surface area, relative simplicity of the synthetic method. 

In contrast to acetylcholinesterase, cholinesterase (acylcholine acyl-hydrolase, butyrylcholinesterase, EC 3.1.1.8) exhibits relatively unspecific esterase activity toward choline esters, with abroad specificity relative to the size of the acyl group. The enzyme is synthesized in the liver and can be found in smooth muscle, adipocytes, and plasma. Its physiological role remains partly obscure, but there is evidence that it is present transiently in the embryonic nervous system, where it is replaced in later stages of development by acetylcholinesterase. It has, therefore, been suggested that cholinesterase functions as an embryonic acetylcholinesterase. 

Table 1. Relative phospholipid specificities for activation of different isoforms of recombinant ATPase II.

With respect to an enzyme, the rate of substrate-to-product conversion catalyzed by an enzyme under a given set of conditions, either measured by the amount of substance (e.g., micromoles) converted per unit time or by concentration change (e.g., millimolarity) per unit time. See Specific Activity Turnover Number. 2. Referring to the measure of a property of a biomolecule, pharmaceutical, procedure, eta, with respect to the response that substance or procedure produces. 3. See Optical Activity. 4. The amount of radioactive substance (or number of atoms) that disintegrates per unit time. See Specific Activity. 5. A unitless thermodynamic parameter which is used in place of concentration to correct for nonideality of gases or of solutions. The absolute activity of a substance B, symbolized by Ab, is related to the chemical potential of B (symbolized by /jlb) by the relationship yu,B = RTln Ab where R is the universal gas constant and Tis the absolute temperature. The ratio of the absolute activity of some substance B to some absolute activity for some reference state, A , is referred to as the relative activity (usually simply called activity ). The relative activity is symbolized by a and is defined by the relationship b = Ab/A = If... 

Therefore, a validation exercise with a variety of compounds with unknown mechanisms of developmental toxicity has only limited value if only used to derive an overall predictability rate of a single assay. It is more useful to elucidate the applicability domain of the assay in terms of the mechanisms of development covered, and to validate that aspect by testing compounds that do or do not affect the applicability domain. For single end point assays such as specific receptor activation assays, this exercise is relatively straightforward. For more complex assays such as those involving embryonic cell differentiation, the understanding of the applicability domain is more complex, as extensive research with the embryonic stem cell test has learned (27, 47). Whole embryo cultures are probably more straightforward in terms of applicability domain as they involve the entire embryo in a limited window of development, but such assays are complex and not animal free. 

Transcriptional activators bind specifically to cognate DNA elements variably located relative to the promoter (see 1.4.2.2) and can interact directly or indirectly with the transcription apparatus. Transcriptional activators depend on the occmrence of regulatory DNA elements for their action and perform their function on specific genes. They are thus termed specific transcriptional activators, to distinguish them from proteins that activate transcription independent of specific DNA elements (see 1.4.3.2). 

The current state of Ser/Thr phosphorylation of a protein is determined by the relative activity of Ser/Thr-specific protein kinase and protein phosphatase. It is therefore imderstandable that the cell has had to develop special mechanisms to balance the two activities with one another, and, when needed, to allow kinase or phosphatase activity to dominate. One of the best investigated examples of coordinated activity of protein kinases and protein phosphatases is the regulation of glycogen metabolism in skeletal muscle. Glycogen metabolism is an example of how two different signals, namely a cAMP signal and a Ca signal meet in one metabolic pathway and control the activity of one and the same enzyme. 

The unmodified pyridine ring, in contrast to imidazole or imidazoline rings, is seldom associated with any specific pharmacophoric activity. It does, however, appear in a number of therapeutic agents due to the fact that benzene rings in biologically active compounds can often be replaced by pyridines in spite of the presence of the basic nitrogen atom. The relatively small number of pyridines discussed below thus do not reflect the frequency of their occurrence in therapeutic agents instead, examples have been selected that illustrate some facet of pyridine chemistry. 

With this approach, the ratios of specific (or marker) catalytic activities in human liver microsomes to cDNA-expressed enzymes provide relative activity factors (RAFs). The RAF provides a means to relate the activity of the cDNA-expressed enzyme to the activity of the enzyme in its native environment. RAF is simply a ratio of enzyme activity or the two systems as used by the investigator. The RAF is calculated as ... 

By using specific catalytic activities, the differences in V ax due to any factor are automatically compensated for in the calculation. Because this method establishes relative contribution, there is no a priori need to use the same units for both components of the RAF calculation as long as the same units are used within the cDNA-expressed enzyme data set and the human tissue data set. Moreover, knowledge of the P450 content in either the cDNA expression system or in human liver microsomes is not necessary in order to make an interpretation. 

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