Environmental control of enzyme synthesis and degradation

The N haplotype of the murine beta-glucuronidase gene is altered in both its systemic regulation and its response to androgen induction

A new haplotype of the beta-glucuronidase gene complex, [Gus]N, has been characterized following its transfer from the PAC/Cr strain to the standard strain C57BL/6J. The N haplotype contains a novel structural gene allele which encodes an allozyme differing from all previously characterized allozymes in both size and charge. Altered systemic regulation is exhibited by the [Gus]N haplotype. Multiple tissues contain levels of GUS protein that are 60 +/- 15% those found in the standard B haplotype. The regulatory mechanism for reduction is complex, involving tissue-specific changes in both enzyme synthesis and enzyme turnover. The changes in GUS protein synthesis do not result from changes in GUS mRNA levels. Instead, the amount of mature enzyme formed per mRNA molecule, or translational yield, is altered. These regulatory changes parallel those seen in other systemic regulatory variants of GUS which are also altered in translational yield. A commonality of mechanism among systemic regulatory variants of this gene is suggested. The N haplotype is also exceptional in the nature of its response to androgenic induction in kidney proximal tubule epithelial cells. The time course for GUS induction consists of a lag period followed by a progressive increase in mRNA, rate of enzyme synthesis, and enzyme activity. For the [Gus]N haplotype the lag is of an exceptionally short duration and the plateau is of a greater magnitude than for any haplotype previously described.

Intrinsic physical instability of the proteins with short half-time: possible role in aging and in carcinogenesis

Different proteins have vastly different lifetime in cells, some survive as long as or even longer than the cell does, whereas others survive for just minutes. Proteins showing short half-times play an extremely important role as cellular regulators, in particular in controlling cell mitosis and in carcinogenesis. These proteins probably possess an intrinsic physical instability, i.e. they are biologically active only while being in a meta-stable non-equilibrium state resulting from the very mechanisms of protein biosynthesis on ribosomes. Aging of these proteins consists of internal equilibration through protein chain folding rather than in accumulation of errors due to intermolecular interactions. We want to attract more attention to such fast (short-lived) proteins. The question of what determines these protein lifetimes is of practical as well as theoretical interest.

Selective depletion of small basic non-glycosylated proteins in diabetes

Degradative rates of small basic non-glycosylated proteins are preferentially enhanced in rat liver cytosol during severe streptozotocin-induced diabetes. Synthetic rates of these classes of proteins are not selectively enhanced in diabetes, so small basic non-glycosylated proteins should be depleted from liver cytosol as a consequence of this disease. To test this hypothesis, proteins were analysed from normal animals, from diabetic animals receiving insulin and from diabetic animals after insulin withdrawal for 3 days. The proteins were separated according to subunit molecular weight by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, according to isoelectric point by isoelectric focusing and according to carbohydrate content by affinity chromatography with concanavalin A linked to agarose. Severe uncontrolled diabetes is associated with the predicted depletion of small basic non-glycosylated proteins from liver cytosol. The preferential degradation and loss of these protein classes may be of considerable physiological importance to the animal.

Nature of RNAs synthesised in rat liver in response to hydrocortisone

Experiments on hybridization of [32P]HnRNA from hydrocortisone-treated rat liver with rat-liver DNA, in competition with unlabelled liver HnRNA from hormone-deficient and treated rats, revealed marked differences in their nucleotide sequence homologies. It was found that the hormone-induced new species of HnRNAs were transcribed both on middle-repetitive and non-repetitive sequences of DNA. However similar differences in the sequence homologies were not detectable in respect of mRNA derived from livers of hydrocortisone-administered and adrenalectomised rats. The results imply that the new species of HnRNA synthesized following hormone treatment may be confined to the nucleus.

Degradation of phenylalanine:pyruvate transaminase after glucagon treatment

Using a single-isotope and immune precipitation technique the half-life (t 1/2) of hepatic phenylalanine:pyruvate transaminase (aminotransferase, EC 2.6.1.--, Number not yet assigned) from glucagon-treated rats was determined to be 2.8 days, similar to that of the control rats (t 1/2 = 3.3 days). The half-life of rat liver total soluble proteins also remained unchanged after glucagon treatment (t 1/2 = 2.7 days in glucagon-treated rats; t 1/2 = 2.8 days in normal). Thus, glucagon has no effect on the degradation of phenylalanine:pyruvate transaminase. Furthermore, the degradation rates are similar for both the holoenzyme and the apoenzyme of phenylalanine:pyruvate transaminase.