Enzymatic synthesis of diastereospecific carbacephem intermediates using serine hydroxymethyltransferase.

The serine hydroxymethyltransferase (SHMT) gene glyA was over-expressed in Escherichia coli and the enzyme was purified to near homogeneity. Reaction conditions for E. coli and rabbit liver SHMTs were optimized using succinic semialdehyde methyl ester (SSAME) and glycine. The catalytic efficiency (kcat/K(m)) of E. coli SHMT for SSAME was 2.8-fold higher than that of rabbit liver enzyme. E. coli SHMT displayed a pH-dependent product distribution different from that of rabbit liver enzyme. For the pyridoxal-5'-phosphate (PLP)-dependent reaction, E. coli and rabbit liver SHMTs showed a high product diastereospecificity. The stoichiometric ratio of PLP to the dimeric E. coli SHMT was 0.5-0.7, indicating a requirement for external PLP for maximal activity. Using SSAME or its analog at a high temperature, E. coli SHMT mediated efficient condensation via a lactone pathway. In contrast, at a low temperature, the enzyme catalyzed efficient conversion of 4-penten-1-al via a non-lactone mechanism. Efficient conversion of either aldehyde type to a desirable diastereospecific product was observed at a pilot scale. E. coli SHMT exhibited a broad specificity toward aldehyde substrates; thus it can be broadly useful in chemo-enzymatic synthesis of a chiral intermediate in the manufacture of an important carbacephem antibiotic.

Enzymatic oxidation of cephalosporin C using whole cells of the yeast Triginopsis variabilis within a "cross-flow filter-reactor".

An economical process for the enzymatic oxidation of cephalosporin C to glutaryl-7-ACA was developed at a pilot plant scale. The process utilized nonviable whole cells of the yeast Triginopsis variabilis containing high levels of D-amino acid oxidase. Prior to use, the whole cells were permeabilized with a 25% acetone/water solution which enhanced their apparent activity by 20- to 50-fold. After permeabilization, the whole cells were incubated at pH 11, which served to selectively deactivate catalase which was present in very large quantities. Deactivation of catalase was critical to achieving high reaction yields. The whole cells were utilized within a "cross-flow filter-reactor" which allowed easy and economical recycle of the cells for repeated use. The overall yield of glutaryl-7-ACA from cephalosporin C was 90-95%. The overall productivity of the yeast was 13 kg cephalosporin C oxidized per kilogram yeast (dry basis). The reaction was run at a concentration of 40 g cephalosporin CL-1 and the overall reactor productivity was 11 g glutaryl-7-ACA l-1 h-1. The process has been thoroughly demonstrated on a 35-l scale, and it should be directly scaleable to 10,000 l or more.

Synthesis and antimicrobial evaluation of 20-deoxo-20-(3,5-dimethylpiperidin-1-yl)desmycosin (tilmicosin, EL-870) and related cyclic amino derivatives.

A series of 20-deoxo-20-cyclic (alkylamino) derivatives of tylosin, desmycosin, macrocin and lactenocin was prepared by reductive amination of the C-20 aldehyde group. The majority of the compounds were prepared using metal hydrides (sodium cyanoborohydride or sodium borohydride) as the reducing agents and a suitable cyclic alkylamine. Subsequently, a more convenient procedure was developed using formic acid as a reducing agent. The C-20 amino derivatives prepared from desmycosin exhibited good in vitro antimicrobial activity against Pasteurella haemolytica and Pasteurella multocida (MIC range of 0.78 approximately 6.25 micrograms/ml) as well as Mycoplasma species (MIC range of 0.39 approximately 6.25 micrograms/ml). Several derivatives showed excellent oral efficacy against infections caused by P. multocida in chicks. One of these derivatives, 20-deoxo-20-(3,5-dimethylpiperidin-1-yl)desmycosin (tilmicosin or EL-870) was selected for development as a therapeutic agent for pasteurellosis in calves and pigs.