Plasma Membrane Marker Enzymes in Developing Sea Urchin Embryos

Abstract

Summary

In sea urchin embryos during the first few hours after fertilization the number of cells increases from 1 to 2¹⁰ and plasma membrane volume should increase about tenfold. Since the increase in plasma membrane biomass might also be accompanied by a proportional increase in membrane marker molecules, presumptive plasma membrane “marker” enzymes were assayed in whole homogenates of sea urchin cultures as well as in plasma membrane-enriched fractions. Adenylate cyclase activity in whole homogenates remained constant (except for a brief postfertilization increase) from the unfertilized egg to the swimming blastula stage. In membrane-enriched fractions the activity of this enzyme increased. Reliable levels for Na⁺-, K⁺-ATPase activity were not obtained; 5′-nucleotidase activity could not be detected. Mg²⁺—ATPase, p-nitrophenylphosphatase, and unesterified cholesterol apparently cannot be used as plasma membrane markers in sea urchins. It was concluded that the increase in plasma membrane biomass of one order of magnitude did not necessarily require an equivalent increase (or de now synthesis) of constituent molecules i.e., membrane enzymes. The latter may exist preformed in the unfertilized egg and thus would be of maternal origin during the early stages of development.