Hang on, Ajinomoto powder is a brand name, and the original brand name under which the synthetic extract was (and still is) marketed. Aji-no-moto: The Root Of Flavor. This is the same extraction that has only recently crossed the Western culinary vocabulary in the form of "umami." Same thing.
That same material is naturally extracted from various substances, the most famous being kombu kelp, which is the basis of most dashi.
There has been debate for a long time as to whether the synthetic extract produces adverse reactions, whether anyone is allergic to it, whether it causes cancer, and so on. But as I understand it, despite lots of anecdotal evidence about people turning red when they eat at Chinese restaurants that use a lot of MSG powder, there has never been a proper study done that showed any of these effects. There was one such study done on rats, but I am reliably informed that the control was faulty: the rats were given vast doses of this material, and were not compared against rats given equally vast doses of some "neutral" substance.
Not that I use MSG powder in my own cooking, but there isn't really any solid evidence against it.