Muscle kinetic constants

Table 3. Whelk Muscle Kinetic Constants of Pyruvate Kinase and Metabolite Levels in Aerobic Versus Anoxic Tissue ...

A careful kinetic study has shown that, although all catalyse the same reaction, both the kinetic constants Km and Km, differ. The kinetic characteristics match the requirements of the tissues, e.g. Fmax is high in skeletal muscle but low in heart muscle. 

Sato et al. (1991) expanded their earlier PBPK model to account for differences in body weight, body fat content, and sex and applied it to predicting the effect of these factors on trichloroethylene metabolism and excretion. Their model consisted of seven compartments (lung, vessel rich tissue, vessel poor tissue, muscle, fat tissue, gastrointestinal system, and hepatic system) and made various assumptions about the metabolic pathways considered. First-order Michaelis-Menten kinetics were assumed for simplicity, and the first metabolic product was assumed to be chloral hydrate, which was then converted to TCA and trichloroethanol. Further assumptions were that metabolism was limited to the hepatic compartment and that tissue and organ volumes were related to body weight. The metabolic parameters, (the scaling constant for the maximum rate of metabolism) and (the Michaelis constant), were those determined for trichloroethylene in a study by Koizumi (1989) and are presented in Table 2-3. 

A. Horska and R. G. Spencer, Measurement of spin-lattice relaxation times and kinetic rate constants in rat muscle using progressive partial saturation and steady-state saturation transfer. Magn. Reson. Med., 1996, 36, 233-240. 

M. Stoye-Herzog and W. Ulbricht, unpublished results). Much lower values are deduced from kinetics as Kd=kb/k with k the forward, kb the backward rate constant of the veratridine-receptor reaction. For frog muscle k[a=0.3 s"V2xlO Af" s" =0.15 pM has been reported, an unusually low value for 7°C (Leibowitz et al. 1986). Another low value was determined for ventricular myocytes, however, coactivated with BDF 9145, as fCd=0.51 pM (see Sect 3.1 Wang et aL 1990). This value compares with half-saturating veratridine concentrations of 3.5 and 60 pM as derived from tail currents, at the end of 50-ms pulses, in this preparation in the presence and absence of coactivator (Zong et al. 1992). A possible explanation for these discrepancies in Kd is presented in Sect. 7.2. 

In kinetic analysis, the rate constants of both the dephosphorylation and relaxation were determined in several cases in bovine tracheal muscle after 15 s of electrical stimulation, at 37°C, LC20 was dephosphory-lated with an apparent first-order rate constant (fc) of... 

The rate constant for turnover of the slow component was 0.13 0.03/ day (mean SEM., n = 10) from which a value of turnover of the phosphorylase pool can be calculated as 0.1/day (Beynon etal, 19%). This compares well with values of 0.12/day obtained for gastrocnemius muscle (Leyland et ai, 1990) and 0.13/day for total hind limb and back muscle (Leyland and Beynon, ). The fast pool (presumed to be all labile forms of the vitamin) was turning over very quickly, with a rate constant of 1.3 0.4/day (a half-life of 12 hr). However, the experimental protocol that we use does not permit acquisition of a sufficiently detailed data set to acquire accurate kinetics on the fast pool. This preliminary analysis of the data also implies that the fast pool accounts for about 50% of the total vitamin B6 in the body—it is not yet clear whether this is consequential to the inability to define the fast phase with a high degree of precision or whether the muscle phosphorylase itself partitions into two pools that differ in accessibility. For example, enzyme bound to the glycogen particle might be more stable than enzyme free in the sarcoplasm. Further work is needed to resolve these issues. 

To accelerate the procedure for bioconcentration tests, some investigators (8, 82, 83y 94) express bioconcentration factor as the ratio of the uptake rate constant to the clearance rate constant k2 in fish. In a flowing-water system, Needly et al. 83) found that the log(BCF) of some stable organic compounds determined from the kinetic rate constants in rainbow-trout muscle can be related to log The regression equation between the observed BCF and the calculated Aow values is... 

In an experiment with baby rabbit heart muscle tissue [6], the following kinetic results were obtained (a) for creatine phosphate, CrP, of concentration 1.8 mM the apparent reaction rate was 1.75 xM min and (b) for CrP of concentration 0.35 mM the apparent rate was 0.80 xM min . Calculate the Michaelis constant, /Cm, and the maximum reaction rate, l/max, for this reaction. 

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