Plenty of people know that there’s a gas given off by ripe fruit that can itself accelerate ripening in others – the “banana in a bag” technique. That gas is of course ethylene, identified as such in plants in the early 20th century, and the more chemistry you know, the odder it seems that it should be some sort of plant hormone. It’s small and (as noted) gaseous at room temperature and pressure (its boiling point is nearly -104 C). There are other gaseous signaling molecules even in mammals (nitric oxide, hydrogen sulfide, carbon monoxide) and those I find pretty odd, too, but they at least aren’t as chemically featureless as ethylene. And calling all of these gaseous misses the point a bit, because they’re dissolved in living tissue and associated fluids when they do their work.
So it would be useful to have a way to detect ethylene inside a living cell, but until now there’s been no good way to do that. This new paper, though, has an ingenious solution that’s taken straight from synthetic organic chemistry: olefin metathesis. Hey, it works just fine with ethylene, it’s just that it’s not the sort of reaction (Mo- or Ru-catalyzed) that you’d think of as being compatible with the innards of a living system. But there at right is the ruthenium intermediate that does the trick. This is midway through the metathesis mechanism, and it’s waiting for a reactive olefin to come along for the rest: enter ethylene. And the Ru compound itself is not fluorescent (quenched by the metal), but the BODIPY metathesis product is, and there’s your cell assay! Other organometallic-based fluorescent assays using this effect have been reported for the gaseous neurotransmitters (here’s a review), but this is certainly a neat application of another one.
Ethylene gets made from S-adenosyl methionine (SAM) through an enzymatic pathway involving ACC-oxidase, and there’s a family of transmembrane ethylene receptor proteins on the receiving end. As far as is known, it’s produced in a number of plant tissues, but a system like this one can put that to a real-time test. It’s going to need a bit more work, though. The authors show that the compounds do get into cells (HEK293T at right) and do respond to ethylene (panel B, figure from the paper’s freely available SI), but it looks like there’s a slow background increase of fluorescence even without ethylene present (that is, in mammalian cells that don’t produce any).
This is presumably due to breakdown of the organometallic end of the molecule, with gradual release of the ruthenium. It’s also might be trickier to get these into plant cells than into mammalian models, what with the cell wall and all, but the authors show it working in a model organism (the workhorse algae Chlamydomonas reinhardtii) which has a cell wall and also has chlorophyll (which as you’d imagine can interfere with optical measurements like this). The assay window isn’t large, but it’s there, and I’d guess that structural tweaking of these compounds could lead to better detection and higher metal-center stability, although with that latter factor you’re going to be balancing reactivity with the ethylene. The key will be getting enough signal to see endogenous ethylene production, and the paper says that they’re working on that right now. Good luck to them!