NAPQI
NAPQI (N-acetyl-p-benzoquinone imine) Is a very reactive biproduct of the metabolism of paracetamol via the P-450 system. It is toxic due to it binding to essential protiens inside the hepatic cells causing liver damage.
It is produced when levels of glutathion in the liver cells are low.
References:
http://www.vetmed.vt.edu/education/Curriculum/VM8304/vet%20pathology/CASES/CELLINJURY2/PAGE3-2.htm