Piperine is the phytonutrient responsible for the pungency of black pepper.

Studies show that piperine may enhance bioavailability of curcumin by 2000% in humans.

References

Bhardwaj RK, Glaeser H, Becquemont L, Klotz U, Gupta SK, Fromm MF (2002) Piperine, a major constituent of black pepper, inhibits human P-glycoprotein and CYP3A4. J. Pharmacol. Exp. Ther. 302(2):645-50. PMID 12130727