Influence of pH, nitrogen and phosphorus sources on the production of hepatitis B virus pre-S2 antigen by Hansenula polymorpha.

Experimental design techniques were used to study the influence of the composition of the culture medium on the production of hepatitis B virus pre-S2 antigen by the methylotrophic yeast Hansenula polymorpha. pH, phosphoric acid, ammonium chloride and yeast extract concentrations were selected as experimental factors and their influence was investigated using Central Composite design techniques. The results indicated that antigen yield was maximized at high pH and in a culture medium containing both ammonium chloride and yeast extract. Phosphoric acid was found to have a detrimental effect on antigen production. This study allowed a 50% increase in antigen production in a medium in which the yeast extract concentration was decreased to 32 g/l. These optimal conditions have been confirmed with an octagonal design experiment. Moreover, it was shown that the antigen produced was very stable up to at least 8 days after induction and that the yeast extract concentration could be lowered to 22 g/l without appreciable effect on antigen yield. The increase in antigen production was not due to an increase in cell biomass, since no correlation could be found between these two parameters. The newly defined culture medium should allow a greatly increased antigen production at the fermentor level, at a lower cost and with minimal operational problems.