| proactinium | <chemical> Protactinium. A radioactive element of the actinide group of metals. It has the atomic symbol pa, atomic number 91, and atomic weight 231. It decays by alpha-emission. Chemical name: Protactinium (12 Dec 1998) |
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| proactivator | <biochemistry> A substance that, when chemically split, yields a fragment (activator) capable of rendering another substance enzymatically active. (05 Mar 2000) |
| proactive inhibition | The state or process hypothesised to account for poorer learning rate for elements later in a series as compared to the learning rate for elements coming earlier in a series. (12 Dec 1998) |