Hexa- and pentapeptide extension of proalbumin: feedback inhibition of albumin synthesis by its propeptide in isolated hepatocytes and in the cell-free system

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Abstract
Addition of the chemically synthesized proalbumin hexapeptide in a concentration of 110 .mu.M to the medium of isolated rat hepatocytes decreased net albumin synthesis by 12%. The synthesis of other secretory proteins was not altered. A weaker inhibitory effect on albumin synthesis was found for a tetrapeptide, a possible degradation product of the proalbumin hexapeptide. For the uptake of the hexa- and tetrapeptide into the cells, bovine serum albumin is required. In a [rabbit] reticulocyte and in a wheat germ cell-free system a propeptide concentration of 600 .mu.M inhibited albumin synthesis by 50%, whereas total protein synthesis was inhibited by 19% only and the synthesis of .alpha.1-antitrypsin was not inhibited. The synthesis of preproalbumin may be regulated by a feedback mechanism with its propeptide as inhibitor.