Human valacyclovir hydrolase/biphenyl hydrolase-like protein is a highly efficient homocysteine thiolactonase.

Judit Marsillach,Edward J Hsieh,Rebecca J Richter,Peter M Rademacher,Stephanie M Suzuki,Clement E Furlong,Matthew G Mcdonald,Allan E Rettie,Michael J Maccoss,Fernando Rodrigues-Lima

doi:10.1371/journal.pone.0110054

Judit Marsillach, Edward J Hsieh + Show 8 more

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https://doi.org/10.1371/journal.pone.0110054

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Abstract

Homocysteinylation of lysine residues by homocysteine thiolactone (HCTL), a reactive homocysteine metabolite, results in protein aggregation and malfunction, and is a well-known risk factor for cardiovascular, autoimmune and neurological diseases. Human plasma paraoxonase-1 (PON1) and bleomycin hydrolase (Blmh) have been reported as the physiological HCTL detoxifying enzymes. However, the catalytic efficiency of HCTL hydrolysis by Blmh is low and not saturated at 20 mM HCTL. The catalytic efficiency of PON1 for HCTL hydrolysis is 100-fold lower than that of Blmh. A homocysteine thiolactonase (HCTLase) was purified from human liver and identified by mass spectrometry (MS) as the previously described human biphenyl hydrolase-like protein (BPHL). To further characterize this newly described HCTLase activity, BPHL was expressed in Escherichia coli and purified. The sequence of the recombinant BPHL (rBPHL) and hydrolytic products of the substrates HCTL and valacyclovir were verified by MS. We found that the catalytic efficiency (kcat/Km) of rBPHL for HCTL hydrolysis was 7.7 × 104 M−1s−1, orders of magnitude higher than that of PON1 or Blmh, indicating a more significant physiological role for BPHL in detoxifying HCTL.

Highlights

Elevated homocysteine (Hcy) levels have been associated with cardiovascular disease [1,2], diabetic retinopathy [3], birth defects, osteoporosis, renal failure [4] and Alzheimer’ disease (AD) [5,6]
Hcy can be synthesized in all human organs, most of it is detoxified in the liver and kidneys, organs that possess all the enzymes involved in the methionine metabolism [7]
We report the purification of a human hepatic enzyme that demonstrates high Homocysteine thiolactonase (HCTLase) activity and was identified as biphenyl hydrolase-like protein (BPHL) by mass spectrometry (MS)

Summary

Introduction

Elevated homocysteine (Hcy) levels have been associated with cardiovascular disease [1,2], diabetic retinopathy [3], birth defects, osteoporosis, renal failure [4] and Alzheimer’ disease (AD) [5,6]. A homologue of the amino acid cysteine, is generated from methionine following demethylation and can be converted back to methionine or to cysteine. Hcy serves as a precursor for maintaining the levels of methionine and cysteine. Hcy is methylated through two enzymes from the methionine and folate cycles with the aid of the co-factor vitamin B12. If there is an excess of methionine, Hcy is converted to cysteine via transsulfuration, which requires vitamin B6. Hcy can be synthesized in all human organs, most of it is detoxified in the liver and kidneys, organs that possess all the enzymes involved in the methionine metabolism [7]. Hcy is converted to homocysteine thiolactone (HCTL) through errorediting reactions catalyzed by methionyl tRNA-synthetase.

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