The molecular weight profile of enkephalin-containing peptides in the sheep adrenal gland changes during development

We have measured the content of enkephalin-containing peptides (ENK-containing peptides) in adrenal gland extracts from fetal sheep between 68-142 days gestation and from adult sheep. We investigated whether there are changes in the post-translational processing of ENK-containing peptides in the fetal sheep adrenal gland with increasing gestational age. ENK-containing peptides in adrenal extracts from fetal sheep (68-142 days gestation) and adult sheep were separated using gel filtration chromatography and the ENK immunoreactivity (ENK-IR) was measured using RIAs for Met-enkephalin-Arg6,Phe7 immunoreactivity (MERF-IR) and Met-o-enk immunoreactivity (MET-O-ENK-IR). The MET-O-ENK-IR in fetal and adult sheep adrenal extracts was distributed in four main peaks, which corresponded to mol wt (MW) ranges of more than 12, 7-12, 3-7, and less than 3 kDa. There was a significant increase (P less than 0.05) in the total ENK-IR content of the fetal adrenal between 68-78 (42.4 +/- 19.2 ng/adrenal) and 100-121 (320.1 +/- 142.6 ng/adrenal) days gestation. There was also a significant increase in the proportion of MET-O-ENK-IR and MERF in the less than 3 kDa range between 68-78 (MET-O-ENK, 6.8 +/- 2.7%; MERF, 10.3 +/- 1.4%) and 100-121 (MET-O-ENK, 24.4 +/- 5.1%; MERF, 26.2 +/- 5.8%) days gestation, with an associated decrease in the ratio of ENK-containing peptides in the high MW form (i.e. greater than 3 kDa) compared to those in the low MW (less than 3 kDa) forms (10.4 +/- 2.5 at 68-78 days gestation; 4.2 +/- 1.1 at 100-121 days gestation). There was also a significant decrease in the percentage of MET-O-ENK in the 12 kDa range after 139 days gestation (125-135 days gestation, 20.4 +/- 2.2%; 139-142 days gestation, 4.0 +/- 2.1%). Therefore, there is an increase in the proportion of the low MW forms of ENK-containing peptides in the fetal sheep adrenal with advancing gestational age, which may reflect changes in the post-translational processing of the precursor proenkephalin-A during development.