The lipoprotein composition of amniotic fluid.

The lipoprotein composition of amniotic fluid (AF) has been studied and it was found that it consists of high density lipoprotein (HDL) only. At about the 26th week of pregnancy the HDL content reaches a maximum and decreases thereafter. On crossed immunoelectrophoresis the HDL of AF was separated into two components if the agar layer contained an antiserum against apolipoprotein A-I. The component with the higher electrophoretic mobility (but not the slower moving component) could also be visualized with an antiserum against HDL. When maternal or umbilical cord serum (UCS) was analysed by crossed immunoelectrophoresis using the anti-apoA-I antiserum, we found that they only contained the faster moving lipoprotein component. If UCS was treated with phospholipase A2 and then subjected to crossed immunoelectrophoresis, the slower moving component appeared at the expense of the faster one. Likewise when UCS was incubated with AF from the 15th week of pregnancy or later, the lipoprotein component of UCS with the high mobility was partially converted to the component with the slower mobility. It is concluded that AF probably contains a phospholipase, an enzyme which may promote the onset of labour.