Thermostability of human alpha-L-fucosidase. Relationship to fucosidosis and low-activity serum alpha-L-fucosidase.

Thermostability studies have been performed at different preincubation temperatures (37-65 degrees C) on human alpha-L-fucosidase (alpha-L-fucoside fucohydrolase, EC 3.2.1.51), purified serum and liver enzyme, the isoelectric forms of purified liver enzyme which were separated by preparative isoelectric focusing, crude adult and fetal liver supernatant enzyme and neuraminidase-treated enzyme. Very different thermostability curves were found for the various isoelectric forms of alpha-L-fucosidase. The most neutral form (I) is least thermostable and the most acidic form (VIII) most thermostable, with the intervening forms (II-VII) having intermediate thermostabilities. For the isoelectric forms of liver alpha-L-fucosidase there appars to be a significant trend of increasing thermostability with increasing acidity (and presumably, increasing amounts of sialic acid). In order to determine what role, if any, sialic acid plays in determining the thermostability of alpha-L-fucosidase, comparative thermostability studies were performed on alpha-L-fucosidases from different human tissues which are reported to contain varying amounts of sialic acid. The purified sialic acid-rich serum enzyme is considerably more thermostable than the purified liver enzyme. The fetal liver enzyme (which is less acidic and may contain less sialic acid than the adult liver enzyme) is less thermostable than adult liver alpha-L-fucosidase. In contrast to all of the above findings which suggest that sialic acid confers thermostability to alpha-L-fucosidase, neuraminidase treatment of human liver alpha-L-fucosidase did not change its thermostability, even when considerable desialylation occurred as monitored by isoelectric focusing. The reason for these apparently inconsistent findings is not clear at the present time but several possible interpretations of the data are given.