Activators threshold concentration

After allelochemicals have been identified in the substrate, concentrations should be calculated, and threshold concentrations for activity should be determined against test plants using combinations of compounds present in the substrate, in addition to individual ones. Undoubtedly, many important allelopathic effects have been overlooked because of the use of single allelochemicals in determining threshold concentrations for activity. 

Fleisher reported that both sarin and VX increased the sensitivity of the isolated frog s rectus abdominis to external application of acetylcholine and at the same time decreased the activity of cholinesterase in the external surfaces of the muscle cells. Sarin at 5 x 10-7 M reduced the threshold concentration of acetylcholine for inducing contraction of the muscle to 8% of that required before application of sarin. The same concentration of VX reduced the threshold concentration of acetylcholine to 6.7% of that needed previously. Contemporaneously, the activity of cholinesterase in the external surfaces of the muscle cells was reduced to 8.1% and 0% of that before application of sarin and VX, respectively. Addition of 2-PAM I at 5 x 10 M to the baths in which the muscles were suspended had little effect on the activity of the enzyme in homogenates of the muscles, but restored 75% and 91%, respectively, of the activity of cholinesterase in the external surfaces of the muscles exposed to sarin and VX. At the same time, the concentration of acetylcholine required to induce contraction of the muscles was raised to 53.3% and 58.3% of the original threshold concentrations, respectively, for the muscles exposed to sarin and to VX. 

A variety of responses can be initiated by the direct interaction of metals with cellular components. Membrane damage and enzyme inhibition are examples of such a metal effect. Above a certain threshold concentration of metals in the cell, its physiological state is irreversibly changed (Van Assche et al., 1988). This response is reflected by an increase in capacity (activity under non-limiting substrate and coenzyme concentrations) of certain enzymes. This increase in capacity is generally called enzyme induction. These secondary, indirect effects of metals are considered to play an important role in the stress metabolism induced by toxic metal concentrations. 

For the majority of olefin polymerisations with heterogeneous Ziegler Natta catalysts, the polymerisation rates, Rv, are proportional to the concentrations of procatalyst (MtX ) and monomer (M), but do not depend on the concentration of alkylaluminium activator (A) as long as a threshold concentration is maintained [37] ... 

In many ways, colour, which is a sensation, should be assessed visually. On analogy with the treatment of odorants, Hofmann73 has taken a crucial step in this area by defining a colour dilution factor (CD) and a colour activity value (CAV). CD is the factor required for any solution of a colorant x to be diluted to its colour threshold. CAVX is the ratio of the concentration of x (jug kg-1) to its threshold concentration (jug kg-1)- The colour contribution of a component colorant to the colour of a mixture can then be defined as... 

Derivation of threshold concentrations of sensory-active compounds... 

Barium hydroxide hydrolysis followed by amino acid analysis revealed a sulfated Tyr residue. Subsequently, the myotropic nature of PSK was demonstrated on the isolated hindgut preparation of P. americana where the threshold of activity concentration was determined to be 0.25 nM, virtually the same as the threshold concentration (0.22 nM) of LSK on the isolated L. maderae hindgut (13). In addition to PSK, the non-sulfated form of LSK-II was isolated and structurally characterized from the P. americana cc extracts, but the biologically active sulfated form was not found (19). 

The octapeptide leucopyrokinin (LPK) was the eleventh and final myotropic peptide isolated from L. maderae (11). Unlike the leucokinins and LSK s, LPK exhibits a myotropic activity on the muscles of the cockroach foregut and oviduct (Holman, G. M. and Nachman, R. J., unpublished observation.). LPK was the highest titered of the Leucophaea myotropins (1.36 pmol/head) but was the least potent on the isolated hindgut preparation (threshold concentration -0.6 nM). LPK shares a 50% sequence identity with Pea-HTH-II at the 1, 4, 5, and 6 positions and is amidated at the C-terminus. Yet LPK does not contain Trp, which is present at position 8 in every AKH/RPCH peptide. Furthermore, LPK is positively charged with a residue of Arg at position 8. These structural observations show that LPK is not a member of the AKH/RPCH family. A study of LPK analogs supports this view (20). 

Based upon the observations of Nachman et al. (20), we predicted that the PBAN s would stimulate the isolated visceral muscles of Leucoohaea. This prediction was correct. Hez-PBAN (Nle, Nle ) stimulated the contractions of the isolated Leucoohaea hindgut above a threshold concentration of 14.5 4.2 nM (Holman, G. M. and Nachman, R. J., unpublished.). At threshold concentration, Hez-PBAN (Nle, Nleu ) was about four-fold less active than LPK on the isolated Leucoohaea oviduct (6.3 2.5 nM vs. 

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