14C radiolabeling of proteins to monitor biodistribution of ingested proteins

The economical preparation of microgram quantities of 14C-labeled proteins by in vacuo methylation with methyl iodide is described. The 14C radiolabeling was achieved by the covalent attachment of [14C]methyl groups onto amino and imidazole groups by reaction in vacuo with [14C]methyl iodide. The method was tested by investigating the biodistribution of 14C in rats that were fed 14C-labeled human soluble cluster of differentiation 14 (CD14) protein, a receptor for bacterial lipopolysaccharide. Two other control proteins, bovine serum albumin (BSA) and casein, were also labeled with 14C and used for comparative analysis to determine the following: (i) the efficacy and cost efficiency of the in vacuo radiolabeling procedure and (ii) the extent of incorporation of the 14C label into the organs of orogastrically fed 10-day-old Sprague–Dawley rats. [14C]BSA, [14C]casein, and [14C]CD14 were individually prepared with specific radioactivities of 34,400, 18,800, and 163,000 disintegrations per minute (dpm)/μg, respectively. It was found that the accumulation of 14C label in the organs of [14C]CD14-fed rats, most notably the persistence of 14C in the stomach 480 min postgavage, was temporally and spatially distinct from [14C]BSA and [14C]casein-fed rats.