I was just talking about this the other day with ebola...
So there actually exist companies that will sell you custom plasmids. Plasmids are little circular loops of DNA that cells can read and make protiens from. Little programs for cells, basically. The plasmids contain a signal to tell the cell "this is good DNA", a gene for antibiotic resistance, sometimes a fluorescent protien like GFP, and your "target code".
Let's say we want to express CYP2D6 in E.coli. We get some E.coli to start from, some culture medium, and our plasmids. Then you do some trickery with a calcium chloride solution and temperature shocking the cells to 'drive' the plasmids through the cell wall and into the cells. Now that your cells are programmed, put them in a petri dish full of medium treated with antibiotics. The cells that have your foreign DNA are antibiotic-resistant and all the other cells die.
Now you have a bunch of colonies on a plate, and you have to scale up. So you get a temperature controlled vat fermentation reactor, charge it with food for your ecoli, and warm it all up with vigorous mixing. Eventually you'll have cells expressing your protien. You can either express protiens that will directly synthesize drug components, or in this case, proteins you can use as reagents.
Now you take your ugly soup full of bacteria and waste products, add antifoaming agents, and filter it all. Then seperate your several gra,ms of protien from the hundreds of litres of fermentation stock by ion exchange chromatography or something. Now you can mix your protien with a little NADH and have it demethylate your favourite drugs.
Problems: It's slow, messy, requires a huge infrastructure investment, doesn't always work, and yields are poor.
Advantages: You can make very specific protiens like THC Synthase and shit be expressed in biological systems and "work right away".
