Human α-L-Iduronidase

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Abstract
The physicokinetic and immunologic properties of the purified low and high uptake forms of the human lysosomal hydrolase, a-L-iduronidase, have been determined and compared. The apparent K(m) and V(max) values for the low and high uptake forms were similar toward two artificial substrates, 4-methylumbelliferyl-α-L-iduronide (0.07 and 0.06 mmol/1; 16.15 and 14.85 pmol/min/mg, respectively), and phenyl-α-L-iduronide (1.42 and 1.66 mmol/1; 0.83 and 1.05 μmol/min/mg, respectively), and one natural substrate, anhydro-[^3 H]- mannitol-iduronide (0.86 and 1.04 mmol/1; 2.50 and 2.79 μmol/min/mg, respectively). The pH optima for both purified forms also were similar for each of the three substrates (~ 3.50, ~3.50, and ~4.50, respectively). Heparin markedly inhibited the 4-methylumbelliferyl-a- L-iduronide activities of both the low and high uptake forms, while dermatan sulfate and heparan sulfate were more inhibitory toward the low uptake activity. EDTA was a potent inhibitor of both enzyme forms; the divalent cations, Mg^2+ and Ca^2+, could recover up to 30% of the enzymatic activities after EDTA treatment. ρ-Chloromercuribenzoate and maleate also were inhibitory, whereas dithiothreitol and 2-mercaptoethanol were stimulatory. Both enzyme forms had similar thermostabilities; the half-lives at 45, 52, and 60 °C were about 38, 24 and 12 min, respectively. The low and high uptake forms were immunologically crossreactive as demonstrated by Ouchterlony double immunodiffusion and immunotitration studies using anti-human low uptake antibodies.

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