Abstract
Using rT3 as substrate, an in vitro 5′D assay was validated for use with liver tissue from adult Japanese quail, by defining conditions under which activity is proportional to enzyme (protein) concentration and is linear with incubation time. Activity was measured as the release of ^125^I from labeled rT3. Using validated assay conditions we found the following 5′D characteristics: maximal activity from 10 to 50 mM dithiothreitol (cofactor), an apparent K~m~ of 0.52 μM rT3, pH optimum of 7.6‐8.5, complete inhibition by 1 mM propylthiouracil and by 1.0 mM iopanic acid, and substrate “preference” of rT3 > T4 > T3. Based on these characterizations the quail hepatic 5′D activity is like the Type I 5′D activity found in mammalian liver and kidney and embryonic chicken liver. To determine how previous unvalidated assays, that used high tissue and relatively low substrate (T4) concentrations, influenced 5′D studies we reevaluated 5′D development using an assay validated for each developmental stage with rT3 as substrate. We found extreme quantitative differences in the activities measured and in the proportional relationships between stages, and only limited qualitative similarity in the pattern of 5′D development when unvalidated T4 assay results were compared with validated rT3 assay results. Our data in this paper show good correspondence between whole liver 5′D activity per unit body weight and plasma T3/T4 ratios for the developmental stages sampled.