Triacetylated insulin: biologic activity and resistance to degradation

Tritiated N-hydroxysuccinimide acetate was prepared with specific activities up to 5 Ci/mmole and utilized to prepare tritiated triacetyl insulin. Binding of triacetyl insulin to liver plasma membranes was measured by its capacity to displace 125I-monoiodoinsulin. At low concentrations, less than 10 ng/ml triacetyl insulin appears to be as effective as native insulin in reducing the binding of 125I-monoiodoinsulin to plasma membranes. At concentrations of 20 ng/ml and higher, triacetyl insulin is significantly less effective than native insulin in displacing binding of 125I-monoiodoinsulin to plasma membranes. The properties of triacetyl insulin in this system are not ascribable to deacetylation and conversion of the substituted product to native insulin. Biologic activity of triacetylated insulin was studied in two other in vitro systmes. A comparison was made of the capacity of native beef insulin and its triacetyl derivative to stimulate glucose oxidation by epididymal fat pads. At all three concentrations tested (2, 6, and 18 ng/ml), triacetyl insulin exerted considerable activity, although its potency was significantly less than that of native insulin. Similar effects were observed when biologic activity was measured by induction of tyrosine-alpha-ketoglutarate transaminase in a cultured liver cell system where significant activity of triacetyl insulin was found at concentrations of 10(-9)-10(-7) M. In all systems tested, the activity of triacetylated insulin could not be accounted for by deacetylation and conversion to native insulin. In all systems studied, triacetyl insulin was more resistant to degradation than was monoiodoinsulin.