A sensitive bioassay for enzymes that synthesize retinoic acid.

Retinoic acid (RA) is a potent regulator of gene transcription and it plays a pivotal role in neural development. As the compound is active at nanomolar concentrations, standard RA detection methods based on high-pressure liquid chromatography (HPLC) are poorly suited for neurodevelopmental questions and single RA measurements require pooling of tissues from very large numbers of embryos. An alternative approach is to determine the potential for RA synthesis by assaying for the enzymes that catalyze the last step of RA synthesis, the irreversible oxidation of retinaldehyde to RA. In a quantitative comparison of retinaldehyde dehydrogenase levels with endogenous RA levels, we found a good concordance between the two parameters. For the detection of retinaldehyde dehydrogenases we have developed an assay which involves analyses of protein fractions, separated by isoelectric focusing (IEF), with a RA responsive cell line; operationally, this technique is a zymography bioassay. This method is exquisitely sensitive: tissue volumes as small as a sugar grain can be assayed, and it can be used on all species. Separation by IEF allows in a single run the identification and relative quantitation of several enzyme isoforms.