COMPARISON OF THE THERMOSTABILITY PROPERTIES OF 3 ACID-PHOSPHATASES FROM MOLDS - ASPERGILLUS-FUMIGATUS PHYTASE, ASPERGILLUS-NIGER PHYTASE, AND ASPERGILLUS-NIGER PH 2.5 ACID-PHOSPHATASE

Enzymes that are used as animal feed supplements should be able to wit hstand temperatures of 60 to 90 degrees C, which may be reached during the feed pelleting process. The thermostability properties of three h istidine acid phosphatases, Aspergillus fumigatus phytase, Aspergillus niger phytase, and A. niger optimum pH 2.5 acid phosphatase, were inv estigated by measuring circular dichroism, fluorescence, and enzymatic activity. The phytases of A. fumigatus and A. niger were both denatur ed at temperatures between 50 and 70 degrees C. After heat denaturatio n at temperatures up to 90 degrees C, A. fumigatus phytase refolded co mpletely into a nativelike, fully active conformation, while in the ca se of A. niger phytase exposure to 55 to 90 degrees C was associated w ith an irreversible conformational change and with losses in enzymatic activity of 70 to 80%. In contrast to these two phytases, A. niger pH 2.5 acid phosphatase displayed considerably higher thermostability; d enaturation, conformational changes, and irreversible inactivation wer e observed only at temperatures of greater than or equal to 80 degrees C. In feed pelleting experiments performed at 75 degrees C, the recov eries of the enzymatic activities of the three acid phosphatases were similar (63 to 73%). At 85 degrees C, however, the recovery of enzymat ic activity was considerably higher for A. fumigatus phytase (51%) tha n for A. niger phytase (31%) or pH 2.5 acid phosphatase (14%). These f indings confirm that A. niger pH 2.5 acid phosphatase is irreversibly inactivated at temperatures above 80 degrees C and that the capacity o f A. fumigatus phytase to refold properly after heat denaturation may favorably affect its pelleting stability.