NADPH is the reduced form of NADP+. This simply means that NADPH has an extra hydrogen ion.
NADPH is formed when upon the reduction of NADP+ . NADP stands for nicotine amide dinucleotide phosphate. It is a coenzyme that is involved in a wide range of anabolic reactions.
During the synthesis of carbohydrates in plants, NADPH acts as a reducing agent, likewise in the synthesis of nucleic acids and lipids.
As a reduced compound, NADPH becomes a molecule with high energy thus being especially useful in driving the Calvin cycle.
Yes it can kill brain cells. if you hold your breath for a short amount of time example 10 seconds it won't do any harm. but if you hold your breath for an extended time like a minute the cells will not get enough oxygen and eventually die. I hope I helped you