Objective: To test the physiological properties of human insulin in which the amino acids Thr (B27) and Pro (B28) are interchanged (PT insulin). This was hypothesised to prevent dimerisation and accelerate the absorption from s.c. tissue without altering the affinity for the insulin receptor. Design: PT insulin was expressed in Pichia pastoris and processed in vitro. The purified compound was used for physiological investigations. Methods: Receptor binding activity to insulin and IGF receptors was evaluated in a competition assay using iodinated PT insulin and recombinant receptors while growth induction properties were evaluated by thymidine incorporation. Absorption kinetics from pig subcutis was investigated by measuring the disappearance of iodinated PT insulin. The potency was evaluated by measuring the blood glucose lowering activity in mice.Results: The absorption of PT insulin was accelerated compared with human insulin, although still slower than Asp (B28) insulin. Human and PT insulin had similar affinities for the human insulin receptor ðK d ¼ 3:6 £ 10 212 vs 5:2 £ 10 212 mol=lÞ while the affinity for the IGF receptor was four times higher for PT insulin than for human insulin ðK d ¼ 3:4 £ 10 28 vs 1:3 £ 10 27 mol=lÞ: This resulted in a slightly higher DNA synthesis when assayed in intermediary insulin concentrations. The blood glucose lowering effect in mice exceeded the effect of human insulin (integral 0-60 min: 61:4^7 vs 30^4; n ¼ 6; P ¼ 0:046). Conclusions: PT insulin is absorbed faster and is more potent than human insulin. Although PT insulin stimulates growth more than human insulin, this will not prevent its use in the clinic, but the main interest will probably focus on investigations to clarify the paradox of full biological activity in connection with the recently described lack of structure in the B-chain.

The effects of treatment for 11 days with human growth hormone (hGH; 140 micrograms/day), thyroxine (T4; micrograms/day) and hGH+T4 on renal growth and content of insulin-like growth factor-I (IGF-I) in hypophysectomized rats have been compared with saline-treated hypophysectomized animals and intact control animals. Right kidney weight and kidney weight/body weight ratio remained low in the saline-treated group (313 +/- 9 mg vs 694 +/- 28 mg in controls on day 11, P < 0.001 and 3.4 +/- 0.12 x 10(-3) vs 4.2 +/- 0.10 x 10(-3), P < 0.005 respectively). In T4- and hGH-treated animals, kidney weight gain was similar (to 420 +/- 14 and 450 +/- 22 mg on day 11 respectively, P > 0.05), whilst the increase was greater in the group given hGH+T4 (to 572 +/- 34 mg, P < 0.001 compared with hGH- and T4-treated groups). The kidney weight/body weight ratio became normal in the T4- and hGH+T4-treated animals but remained low in the hGH-treated group. The renal content of IGF-I was low in the saline-treated animals throughout the study (92 +/- 10 ng/g on day 11 vs 219 +/- 8 ng/g in control animals, P < 0.001), but increased to a maximum of 88% above baseline on day 1 in the group given T4.(ABSTRACT TRUNCATED AT 250 WORDS)

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