Effect of salt stress on crotonobetaine and D(+)-carnitine biotransformation into L(-)-carnitine by resting cells of Escherichia coli

The biotransformation of crotonobetaine and D(+)-carnitine into L(-)-carnitine is affected by salt stress in the resting cells of E. coli 044 K74 and the transformed E. coli K38 pT7-5KE32. A yield of 65 and 80% of L(-)-carnitine, respectively, were obtained with 0.5 m NaCl with the wild and transformed strain compared with the 40% obtained with the control. Higher salt levels reduced the conversion. In L(-)-carnitine transport studies using both strains, the transformed strain presented slightly lower apparent K-m and V values. Arsenate reduced both the transport and biotransformation of crotonobetaine in the presence or absence of 0.5 m NaCl, whereas vanadate only inhibited these processes under salt stress conditions. Hg(II) inhibited both the transport and biotransformation and Pb(II) reduced the biotransformation only under salt stress conditions. Cu(II) produced a significantly higher decrease than Pb(II) in the biotransformation with both substrates in the absence of salt stress conditions, but only affected transport in the presence of such conditions. Furthermore, salt stress affected the CaiT transporter for L(-)-carnitine and crotonobetaine and induced ProU and ProP in the absence of the inducer of the L(-)-carnitine metabolism. It is highly likely that the increase in L(-)-carnitine production was not only due to improved transport but also to the permeabilization effect caused by NaCl, as transport and 1-N-phenylnaphthylamine uptake studies revealed.