Tumors can grow quickly only when they’re well fed, so doctors seek ways to starve the malignancies. Realizing that cancer cells consume more methionine than healthy cells do, researchers engineered a novel human enzyme that degrades the amino acid (ACS Chem. Biol., DOI: 10.1021/cb300335j). In experiments using mice, the protein stopped tumor growth.
Scientists have long recognized that limiting a tumor’s access to methionine could treat cancer. One methionine-limiting strategy uses methionine γ-lyase (MGL), an enzyme from the soil bacterium Pseudomonas putida, which breaks down methionine. When researchers tried injecting rodents and primates with the enzyme, blood levels of the amino acid dropped and tumor growth stopped.
However, the bacterial enzyme causes a strong immune reaction in primates, making it a poor drug candidate, says George Georgiou of the University of Texas, Austin. Also, the enzyme’s half-life in human serum is only two hours, he says. Such a short lifetime would mean patients would have to take larger doses of the enzyme to see any benefit, adds Georgiou.
A human MGL would be a better cancer drug than the bacterial enzyme, Georgiou hypothesized. Unfortunately, no such enzyme exists. He and his team set out to make one.
They selected cystathionine γ-lyase (CGL) as a starting point because it is a human enzyme that closely matches MGL in sequence and catalyzes a similar chemical reaction. Plus, CGL has a longer half-life in serum than MGL does. After comparing the sequences of the two enzymes, the researchers realized that they had to make CGL’s active site more hydrophobic to make it interact with methionine. Compared to cystathionine, which is the molecule CGL binds and reacts with, methionine is greasier.
To modify the enzyme’s active site, the researchers created over 2,000 mutated versions of CGL. They then screened the mutants to determine how fast each chewed up methionine and produced methanethiol and α-ketobutyrate. The team monitored the reaction by adding the compound 3-methylbenzothiazolin-2-one hydrazone, which reacts with α-ketobutyrate to produce an ultraviolet-absorbing molecule. The enzyme that was most efficient at catalyzing the reaction differed from CGL by just three amino acids and had a half-life of 78 hours in human serum.
To test the novel enzyme’s effect on tumors, the researchers injected the enzyme into the tail veins of mice three times per week for four weeks. The scientists had grafted human nerve tumors into the rodents. The mice’s blood methionine concentrations were 124 μM at the start of the experiment but dropped to 6 μM after treatment. In mice that didn’t receive the enzyme, the nerve tumors tripled or quadrupled in size over the course of the study. Meanwhile, the tumors of the animals receiving the methionine-busting enzyme did not grow at all.
“It’s very impressive,” says Eugene Frenkel of the University of Texas Southwestern Medical Center. They are “well on their way” to developing a medicine, he says. He thinks the enzyme’s current rate of methionine chewing would work against a wide range of fast-growing tumors. Still, Georgiou wants to increase the enzyme’s speed, which would make a lower dose of the protein able to slow tumor growth.