Dopamine b-Hydroxylase: Determination of half-life and appearance in lymph fluid after IV injection of 125 I-DBH into rats.

A 4 day half-life of dopamine beta-hydroxylase (DBH) was determined for rats injected IV with 125I-rat DBH from the slow exponential component of radioactivity appearing in plasma, urine, feces and combined urine and feces. Half-life estimates for 125I-rat DBH injected IV into WKY and SHR animals did not differ from Sprague Dawley (Zivic Miller) rats. Radioactivity declined in parallel in plasma, urine and feces following IV 125I-rat DBH administration and each radioactivity falloff curve could be resolved into two components. The slow phase of the decline of radioactivity excreted into urine and feces from which DBH half-life was calculated occurred between 5 and 25 days after 125I-rat DBH injection. The early fast phase which is associated with distribution of the exogenous protein in body fluids and tissues continued for approximately the first 140 hr after DBH injection. The distribution characteristics of IV administered active bovine DBH and 125I-rat DBH into the lymphatic system were examined. After active bovine DBH or 125I-rat DBH was injected IV into rats, active DBH or radioactivity, respectively, appeared in lymph fluid (thoracic duct) within 20 min; reached peak concentrations within 90 min, and thereafter, declined in parallel with the plasma concentration. The concentration of radioactivity in plasma and lymph fluid were found to be unequal at 9 hr but were equivalent 68-75 hrs after IV injection of 125I-rat DBH. Based on the amount of active DBH or radioactivity which accumulates in lymph fluid it is clear that a substantial amount (greater than 50%) of the DBH in blood circulates through the lymphatic channels. Analysis of parallel experiments with labelled serum albumin indicate that use of these methods to study plasma proteins do provide sensitive measures of biological half-life and lymphatic distribution characteristics. Specifically for DBH, the results of our study suggest that DBH normally circulates in plasma and lymph fluid with a biological half-life of 4 days.