[Problems in the determination of isoelectric points of beta lactamases].

Analytical isoelectric focusing in polyacrylamide gels and preparative electrofocusing in density gradients may give very different pI determination for the same beta lactamase. This difference seems due to a lack of enzyme migration in some parts of the cross-linked polyacrylamide gel. This migration appears easier in density gradient isoelectricfocusing, and thus this technique yields more significant absolute pI values. Analytical isoelectric focusing should only be used for comparison studies.