The effect of Streptococcus pneumoniae infection on the binding of thyroxine to purified rat liver plasma membranes.

Hepatic plasma membranes were isolated from control and Streptococcus pneumoniae-infected rats in order to determine the effect of S. pneumoniae infection on the binding capacity and affinity of hepatic plasma membranes for T4. Infection did not affect the purity or the yield of isolated membranes. A significant decrease in both total and free serum T4 was observed during infection. Scatchard analysis of membrane binding, determined under optimal conditions, confirmed the presence of high affinity, low capacity sites, as well as low affinity sites for T4 on membranes isolated from both control and infected rats. T4 maximum binding capacity (MBC) of the high affinity sites decreased significantly as the infection became more severe. However, the affinity of these receptors did not change. Neither MBC nor affinity of the low affinity sites was altered by infection. The observed decrease in the MBC of the high affinity binding sites for T4 on hepatic plasma membranes cannot account for the decrease in serum T4 or the hepatic metabolic alterations also known to occur during S. pneumoniae infection.