Dihydroxyacid
dehydratase (DHAD), the rate-limiting enzyme in the synthesis of branched-chain
(BCA) amino acids in bacteria and plants, is sensitive to oxyradical toxicity.
Oxidant stress reversibly inactivates DHAD and causes starvation for BCA and
reversible cessation of growth in Escherichia
coli[1][2].
To better understand the underlying toxicity mechanisms, we have determined the
cellular concentrations of charged-tRNAs for BCA, in E. coli treated with the redox-active chemical, paraquat. Contrary
to expectation, in the paraquat-treated cells, the concentration of only
charged leucyl-tRNA decreased dramatically; whereas, the concentrations of the
other BCAs (valine and isoleucine) increased. This paradoxical result, the
“paraquat effect” can be best explained if leucine is the most abundant amino
acid in the E. coli proteins and
therefore the rate-limiting building block in their synthesis. Based on this
assumption, we investigated the concentration of free amino acids in E. coli and their relative abundances in E. coli proteins. Protein amino acid
frequencies were determined by analyzing one-hundred gene bank protein sequences
with software developed as described in Methods. Leucine is the most abundant
amino acid in the E. coli proteins
(10%) and consequently, the cellular free leucine concentration is smaller and
the native charged-leucyl-tRNA levels are much higher than those of valine and
isoleucine. This has relevance to humans because: leucine-deprivation was shown
to be beneficial in tumor suppression[3], and leucine-supplementation was
beneficial in the recovery from exercise-induced muscle loss [4][5],
and leucine also occurs at a higher frequency in almost all human proteins. In
three human protein categories, we examined it ranged from 9% to 17%. This
predominance of leucine in proteins would make cells vulnerable to impairment
of the leucine pools and could explain our results in E. coli and some of the biological effects of free leucine in humans.
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