Summary
Synthesis of actin is a reasonably correct representation of the platelet production
rate. Measurement of actin production obviates some of the drawbacks encountered in
currently available methods. Platelet actin was characterized on polyacrylamide gel
electrophoreses using methylated radioactive rabbit actin as a reference. Platelet
actin was unambiguously identified by showing that it forms a complex with DNAase-I
similar to the complex obtained with pure rabbit actin.
Six days after intravenous injection of 75Se-seleno-methionine actin was one of the most highly labelled platelet components.
The platelet pellet (one rat per sample) was solubilized with 1% Triton X-100, 0.75
M guanidine-HCl and dialyzed in a medium containing 1% SDS (to eliminate Triton X-100
and guanidine-HCl). A carefully measured aliquot was deposited on a polyacrylamide
gel. After electrophoresis in the presence of SDS the radioactivity of the actin band
was measured and the ratio of the total actin radioactivity to the injected radioactivity
was taken as a measure of platelet production.
The validity of the actin probe was tested with populations of normal animals. The
specific radioactivity of actin was proportional to the injected dose of 75Se-seleno-methionine up to 1 mCi/ kg animal. A semi-log plot of actin specific radioactivity
vs time exhibited a pseudo-first order decrease. Another constituent with a high specific
radioactivity (XM) was excluded as a suitable probe because it was shown to be an adsorbed plasma protein.
Keywords
Platelet production rate - Actin - Actin-DNAase-I complex actin labelling by reductive
methylation - Electrophoresis - Platelet kinetics
