I wrote a while back about the problem of compounds sticking to labware. That sort of thing happens more often than you’d think, and it can really hose up your assay data in ways that will send you running around in circles. Now there’s a report in Science of something that’s arguably even worse. (Here's a good report on it from Bloomberg, one of the few to appear in the popular press).
The authors were getting odd results in an assay with monoamine oxidase B enzyme, and tracked it down to two compounds leaching out of the disposable plasticware (pipette tips, assay plates, Eppendorf vials, and so on). Oleamide is used as a “slip agent” to keep the plastic units from sticking to each other, but it’s also a MAO-B inhibitor. Another problem was an ammonium salt called DiHEMDA, which is put in as a general biocide – and it appears to be another MAO-B inhibitor.
Neither of them are incredibly potent, but if you’re doing careful kinetic experiments or the like, it’s certainly enough to throw things off. The authors found that just rinsing water through various plastic vessels was enough to turn the solution into an enzyme inhibitor. Adding organic solvents (10% DMSO, methanol) made the problem much worse; presumably these extract more contaminants.
And it’s not just this one enzyme. They also saw effects on a radioligand binding assay to the GABA-A receptor, and they point out that the biocides used are known to show substantial protein and DNA binding. These things could be throwing assay data around all over the place – and as we work in smaller and smaller volumes, with more complex protocols, the chances of running into trouble increase.
What to do about all this? Well, at a minimum, people should be sure to run blank controls for all their assays. That’s good practice, but sometimes it gets skipped over. This effect has probably been noted many times before as some sort of background noise in such controls, and many times you should be able to just subtract it out. But there are still many experiments where you can’t get away from the problem so easily, and it’s going to make your error bars wider no matter what you do about it. There are glass inserts for 96-well plates, and there are different plastics from different manufacturers. But working your way through all that is no fun at all.
As an aside, this sort of thing might still make it into the newspapers, since there have been a lot of concerns about bisphenol A and other plastic contaminants. In this case, I think the problem is far greater for lab assays than it is for human exposures. I’m not so worried about things like oleamide, since these are found in the body anyway, and can easily be metabolized. The biocides might be a different case, but I assume that we’re loaded with all kinds of substances, almost all of them endogenous, that are better inhibitors of enzymes like MAO-B. And at any rate, we’re exposed to all kinds of wild stuff at low levels, just from the natural components of our diet. Our livers are there to deal with just that sort of thing, but that said, it’s always worth checking to make sure that they’re up to the job.