Comparative Study of the Properties of the Purified Internal and External Invertases from Yeast

The properties of purified preparations of the internal and external invertases of yeast, designated by their location relative to the cell membrane, show similarities and marked disparities. The molecular weight and the specific activity of the internal enzyme are similar to those of the protein moiety of the external, but their amino acid compositions differ. The external invertase contains cystine or cysteine, and the internal enzyme does not, but neither is activated by cysteine. If the disparities in amino acid composition reflect enzymes that are aggregates of different subunits, at least one of which is identical in the two enzymes, the close similarities of all the other properties would then suggest a precursor-product relationship. The internal enzyme contains little or no carbohydrate, whereas the external invertase is a glycoprotein containing 50% mannan and 3% glucosamine. The latter probably links the protein with the carbohydrate moiety. Both invertases have the same Km for sucrose and raffinose, and neither shows an appreciable transferase activity. The pH optimum for enzymatic activity is the same, but their pH-stability curves differ, the internal invertase showing reversible inactivation at acid pH. The internal invertase has a higher mobility than the external in polyacrylamide gel electrophoresis at pH 6.8 and 8.6. The enzymes are immunologically related, and sucrose-negative mutants lack both external and internal invertase activity.