Abstract
In vivo tracking of the delivery of therapeutic proteins is a useful tool for preclinical studies. However, many labels are too large to use without disrupting the normal uptake, function, or other properties of the protein. Lowโmolecularโweight fluorescent labels allow in vivo and ex vivo tracking of the distribution of therapeutic proteins, and should not alter the protein's characteristics. We tested the in vitro properties of fluorescentโlabeled recombinant human alphaโlโiduronidase (rhIDU, the enzyme deficient in Hurler syndrome) and compared labeled to unlabeled proteins. Labeled rhIDU retained full enzymatic activity and showed similar kinetics to nonlabeled rhIDU. Uptake of labeled rhIDU into human Hurler fibroblasts, measured by activity assay, was equivalent to unlabeled rhIDU enzyme and showed an uptake constant of 0.72โnM. Labeled rhIDU was also able to enter cells via the mannose 6โphospate receptor pathway and reduce glycosaminoglycan storage in Hurler fibroblasts. Subcellular localization was verified within lysosomes by confocal microscopy. These findings suggest that fluorescent labeling does not significantly interfere with enzymatic activity, stability, or uptake, and validates this method as a way to track exogenously administered enzyme.