Insulin degradation in serum of a patient with apparent insulin resistance.

A case is presented of a thin diabetic male who was resistant to large doses of sc and im insulin but responded to small dose of insulin given iv. His serum contained an enzyme that degraded [125I]insulin in vitro. We postulate that his apparent insulin resistance was due to inactivation of insulin at the injection site. We propose that the sera of patients with insulin resistance be tested for [125I]insulin-degrading activity as a possible means of identifying patients with this syndrome.