Alkaline phosphatase vs luciferase as secreted reporter molecules in vivo

Secreted alkaline phosphatase (SEAP) and Metridia luciferase (MLuc) are useful reporter molecules in vitro, but little is understood about their usefulness in vivo. In this study, we investigated in vivo activity of recombinant SEAP and MLuc in blood and urine. When SEAP-transfected cells or recombinant SEAP were injected into rats, substantial increase in the level of serum SEAP was observed. In contrast, activity of SEAP was not detected in urine of rats injected with either the SEAP-transfected cells or recombinant SEAP. SEAP activity was also undetectable in urine of SEAP-injected Nagase analbuminemic rats in which glomerular permeability to macromolecules is enhanced. When MLuc-transfected cells were implanted into rats, activity of MLuc was undetectable not only in urine but also in serum. Even immediately after intravenous injection of recombinant MLuc, activity of MLuc was not detected in serum. Subsequent experiments revealed that, in contrast to SEAP, MLuc was rapidly inactivated either by rat serum, fetal bovine serum, or human serum. Albumin was identified as the molecule responsible for the inhibition of MLuc activity. These data elucidated advantages and limitations of secreted reporter molecules SEAP and MLuc under in vivo situations.