Intracellular reaction rates,enzyme activities and biomass yields in Methylomonas L3: growth rate and substrate composition effects

Summary The effects of specific growth rate and medium feed composition on the metabolic reactions of methanol incorporation and oxidation have been studied in carbon-limited, chemostatic cultures of Methylomonas L3. An in situ radioisotopic tracer technique was employed. The in vivo rates of substrate-carbon flow and the corresponding steady-state levels of several key RuMP-type methylotrophic enzymes were determined over a range of dilution rates from 0.19 to 0.41 h^-1 on methanol and methanol/formaldehyde substrates. It was determined that an absolute correlation does not exist between the in vivo specific carbon flux and the in vitro specific activity of any of the key enzymes studied. Oxidation of substrate-carbon via 6-phosphogluconate dehydrogenase is not stringently regulated in this methylotroph and the extent of its operation may be dependent on kinetic factors which make immediate cellular detoxification of formaldehyde imperative. As such, the cyclic oxidation mechanism in this methylotroph does not appear to be coupled to efficient energy utilization, since it was observed that high levels of the cyclic oxidation flux are commensurate with depressed biomass yields.