Inhibition of growth by imadazol(on)e propionic acid: Evidence in vivo for coordination of histidine catabolism with the catabolism of other amino acids

Imidazole propionic acid (ipa), a gratuitous inducer of the histidine-utilization ( hut ) system in Salmonella typhimurium , inhibits the organism's growth on succinate minimal medium. Induction of the hut system is necessary, but not sufficient, to cause inhibition. A study of the ability of single amino acids to relieve ipa-restricted growth suggests that insufficient glutamate is the cause of slow growth. The inhibition of growth by imidazolone propionic acid (iopa), an intermediate in the catabolism of histidine to glutamate, is similar to that by ipa. Studies using 2, 3, 5-triphenyl tetrazolium chloride plates to examine amino acid catabolism suggest that accumulation of ipa or iopa leads to inactivation of aspartate amino-transferase (AAT). This interpretation is supported by studies of an Escherichia coli mutant lacking AAT. The mutant grows poorly on succinate minimal medium, and the poor growth is relieved by the same amino acids that relieve ipa- and iopa-restricted growth. These and other findings are discussed in terms of coordination of the histidine-utilization system with enzymatic activities involved in the catabolism of other amino acids.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47544/1/438_2004_Article_BF00267937.pd

The total quasi-steady-state approximation for fully competitive enzyme reactions

none3The validity of the Michaelis-Menten-Briggs-Haldane approximation for single enzyme reactions has recently been improved by the formalism of the total quasi-steady-state approximation. This approach is here extended to fully competitive systems, and a criterion for its validity is provided. We show that it extends the Michaelis-Menten-Briggs-Haldane approximation for such systems for a wide range of parameters very convincingly, and investigate special cases. It is demonstrated that our method is at least roughly valid in the case of identical affinities. The results presented should be useful for numerical simulations of many in vivo reactions.noneM. PEDERSEN; BERSANI AM; BERSANI EPedersen, MORTEN GRAM; Bersani, Am; Bersani, E

Quasi steady-state approximations in complex intracellular signal transduction networks - a word of caution

Enzyme reactions play a pivotal role in intracellular signal transduction. Many enzymes are known to possess Michaelis\u2013Menten (MM) kinetics and the MM approximation is often used when modeling enzyme reactions. However, it is known that the MM approximation is only valid at low enzyme concentrations, a condition not fulfilled in many in vivo situations. Recently the total quasi steady-state approximation (tQSSA) has been developed for enzymes with MM kinetics. This new approximation is valid not only whenever the MM approximation is, but moreover in a greatly extended parameter range. Starting from a single reaction and arriving at the mitogen activated protein kinase (MAPK) cascade, we give several examples of biologically realistic scenarios where the MM approximation leads to quantitatively as well as qualitatively wrong conclusions, and show that the tQSSA improves the accuracy of the simulations greatly