In vitro and in vivo characterization of recombinant human butyrylcholinesterase (Protexia) as a potential nerve agent bioscavenger

Previous studies in rodents and nonhuman primates have demonstrated that pretreatment with cholinesterases can provide significant protection against behavioral and lethal effects of nerve agent intoxication. Human butyrylcholinesterase (HuBuChE) purified from plasma has been shown to protect against up to 5 x LD50s of nerve agents in guinea pigs and non-human primates, and is currently being explored as a bioscavenger pretreatment for human use. A recombinant form of HuBuChE has been expressed in the milk of transgenic goats as a product called Protexia. Protexia was supplied by Nexia Biotechnologies (Que., Canada) as a purified solution with a specific activity of 600 U/mg. Initial in vitro studies using radiolabeled 3H-soman or 3H-DFP (diisopropyl fluorophosphate) demonstrated that these inhibitors specifically bind to Protexia. When Protexia was mixed with soman, sarin, tabun or VX using varying molar ratios of enzyme to nerve agent (8:1, 4:1, 1:1 and 1:4, respectively), the data indicated that 50% inhibition of enzyme activity occurs around the 1:1 molar ratio for each of the nerve agents. Protexia was further characterized for its interaction with pyridostigmine bromide and six unique carbamate inhibitors of cholinesterase. IC50 and Ki values for Protexia were determined to be very similar to those of HuBuChE purified from human plasma. These data suggest that Protexia has biochemical properties very similar to those HuBuChE when compared in vitro. Together these data the continued development of the goat milk-derived recombinant HuBuChE Protexia as a potential bioscavenger of organophosphorus nerve agents.

Inactivation of tyrosine hydroxylase by pterin substrates following phosphorylation by cyclic AMP-dependent protein kinase

We reported previously that, following phosphorylation by cyclic AMP-dependent protein kinase, tyrosine hydroxylase in rat corpus striatal extracts is inactivated in a time-dependent and apparently irreversible fashion. Removal of low molecular weight substances from these extracts by gel filtration attenuates this inactivation. We tried to determine the identity of endogenous metabolites that promote inactivation of tyrosine hydroxylase under our experimental conditions. In the present study, we report that the reducing co-substrate tetrahydrobiopterin and its analogues promoted this irreversible inactivation. The concentration that produced a 50% loss of activity (at 20 min) of the phosphorylated enzyme was 0.7 microM and that for the unphosphorylated enzyme was 420 microM. Using enzyme purified from a rat pheochromocytoma, we found that tyrosine, alpha-methyl-p-tyrosine, and a 3-iodotyrosine protected the phosphorylated enzyme against the inactivation produced by tetrahydrobiopterin. Catecholamines (dopamine, norepinephrine, epinephrine, and some of their analogues) also nullified inactivation. In contrast, the product of the reaction, dihydroxyphenylalanine, failed to attenuate the inactivation process. We performed several studies to ascertain the mechanism of inhibition by tetrahydrobiopterin. We considered the possibility that it formed reactive free radicals that produced inhibition. Free radical scavengers, however, failed to block the inhibition produced by tetrahydrobiopterin. Superoxide dismutase, catalase, and peroxidase also failed to protect tyrosine hydroxylase against inactivation. Moreover, when the experiments were performed under anaerobic conditions, the inactivation process was unaffected. These results suggest that reactive oxygenated species were not required for inactivation by tetrahydrobiopterin.

Inactivation of tyrosine hydroxylase activity by ascorbate in vitro and in rat PC12 cells

Tyrosine hydroxylase activity is reversibly modulated by the actions of a number of protein kinases and phosphoprotein phosphatases. A previous report from this laboratory showed that low-molecular-weight substances present in striatal extracts lead to an irreversible loss of tyrosine hydroxylase activity under cyclic AMP-dependent phosphorylation conditions. We report here that ascorbate is one agent that inactivates striatal tyrosine hydroxylase activity with an EC50 of 5.9 microM under phosphorylating conditions. Much higher concentrations (100 mM) fail to inactivate the enzyme under nonphosphorylating conditions. Isoascorbate (EC50, 11 microM) and dehydroascorbate (EC50, 970 microM) also inactivated tyrosine hydroxylase under phosphorylating but not under nonphosphorylating conditions. In contrast, ascorbate sulfate was inactive under phosphorylating conditions at concentrations up to 100 mM. Since the reduced compounds generate several reactive species in the presence of oxygen, the possible protecting effects of catalase, peroxidase, and superoxide dismutase were examined. None of these three enzymes, however, afforded any protection against inactivation. We also examined the effects of ascorbate and its congeners on the activity of tyrosine hydroxylase purified to near homogeneity from a rat pheochromocytoma. This purified enzyme was also inactivated by the same agents that inactivated the impure corpus striatal enzyme. Under conditions in which ascorbate almost completely abolished enzyme activity, we found no indication for significant proteolysis of the purified enzyme as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We also found that pretreatment of PC12 cells in culture for 4 h with 1 mM ascorbate, dehydroascorbate, or isoascorbate (but not ascorbate sulfate) also decreased tyrosine hydroxylase activity 25-50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Alkaline isomerization of ferricytochrome c: identification of the lysine ligand

Changes in the visible absorbance spectra of complexes of horse heart cytochrome c hemopeptide 1-65, peptide 66-104, and their guanidinated counterparts are compared with those characteristic of native and fully guanidinated ferricytochrome c over the pH range 7 to 11. Upon raising the pH, the methionine ligand in the guanidinated hemopeptide 1-65.peptide 66-104 complex is replaced by a strong field ligand. By contrast, the methionine ligand in the hemopeptide 1-65.guanidinated peptide 66-104 is replaced by a weak field ligand. These results demonstrate that lysine 13 does not ligate with the heme iron upon isomerization of ferricytochrome c and that the ligand in the horse heart protein is one of the eight lysine residues in the 66-104 segment of the polypeptide, most likely lysine 79.