Depletion of L-asparagine supply and apoptosis of leukemia cells induced by human glycosylasparaginase

Abstract

ReferencesA lysosomal enzyme glycosylasparaginase (GA; aspartylglucosaminidase; EC 3.5.1.26) hydrolyzes the N-glycosidic carbohydrate-to-protein linkage region, aspartylglucosamine, to L-aspartic acid and l-amino-N-acetylglucosamine through a reaction mechanism similar to L-asparaginase. 3 Glycosylasparaginase is transported in active form into various non-neuronal cell types, including Epstein-Barr virus-transformed (EBV) glycosylasparaginase-deficient lymphoblasts, and it effectively corrects the metabolic defects in glycosylasparaginase-deficient cell lines 4 and mice. 5 The finding that human glycosylasparaginase also hydrolyzes L-asparagine to L-aspartic acid and ammonia-like bacterial L-asparaginases 6 without any L-glutaminase activity 3 led us to investigate its potential cytotoxic activity toward leukemia cells that are dependent on their external supply of L-asparagine. We investigated the ability of human recombinant GA to deplete the intra-and extracellular Asn reservoirs in vitro using EBVtransformed glycosylasparaginase-deficient lymphoblasts from an Letters to the Editor 1167 Leukemia aspartylglycosaminuria patient. 4 Lymphoblasts were continuously grown in an RPMI-1640 (HyClone, Logan, Utah, USA) medium supplemented with 15% of fetal calf serum (BioClear, Wiltshire, UK), 2 mM L-glutamine (HyClone) and 1.5-2.0 mM L-asparagine (Fluka Chemie AG, Buchs, Switzerland) and various concentrations of either GA or Erwinia L-asparaginase (Erwinase, ErAII) for 24 h. The kinetic parameters of the enzymes for the hydrolysis of L-asparagine were determined by a spectrophotometric method as described. 7,8 At pH 7.5, the K m of glycosylasparaginase, Erwinia L-asparaginase and Escherichia coli L-asparaginase was 538, 90 and 130 mM and V max 1.49, 16 and 23 mM/min, respectively. One unit of enzyme causes hydrolysis of 1 mmol of L-asparagine per minute under standard conditions.The depletion of the extracellular Asn concentration in the culture medium from 2.3 to 0.2 mmol/l in the presence of 10 or 40 U/l of glycosylasparaginase or 10 U/l of ErAII took 18, 4 and 4 h, respectively. In the presence of 1500 U/l of ErAII, similar Asn concentration decrease occurred within 5 min. The Asn level in the cell culture medium remained unchanged at approximately 2 mmol/l in the absence of either glycosylasparaginase or ErAII (Figure 1a).To determine whether the enzymes can deplete the intracellular Asn reservoirs of EBV-transformed lymphoblasts, we analyzed both the Asn concentration and the enzyme activity inside the cells during the incubation. In the presence of 10 or 40 U/l of glycosylasparaginase or 10 U/l of ErAII in the cell culture medium, the Asn concentration inside the GA-deficient cells first increased within the first 2 h of incubation from the initial approximately 80 nmol/mg protein and then decreased below 6 nmol/mg protein during the next 16-18, 6 or 10-12 h, respectively (Figure 1b). In the presence of 1500 U/l of ErAII in the culture medium, the intracellular Asn concentration decreased from 89 to 14 nmol/m...

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