Isotopically labeled NEDD8  is useful in determining total cellular concentrations of NEDD8 using the protein standard absolute quantification (PSAQ) or related methods.  Highly purified ¹³C/¹⁵N labeled NEDD8 processed for the quantitative removal of glycine and buffer salts which can interfere with chemical and in vitro reactions. The ubiquitin-like protein NEDD8 is conjugated to targets by the NEDD8-specific E1 activating enzyme (AppBp1/Uba3), the Ubc12 E2 enzyme, and the ROC1/Rbx1 RING FINGER E3 ligase.  NEDD8 plays a critical regulatory role in cell proliferation and development, and modifies nearly all members of the cullin family.