When hepatocyte proliferation is stimulated in the liver by partial hepatectomy, messenger RNAs coding for fibrinogen, actin, c-myc and topoisomerase I are rapidly accumulated. We distinguish an early phase of accumulation (0 -3 h after partial hepatectomy) which is also observed after a sham operation for the four genes, and during inflammation produced by Freund's adjuvant in the case of fibrinogen and c-myc genes. The hepatic response to inflammation appears therefore to mimic events characteristic of the GO/Gl transition, such as the accumulation of the c-myc mRNA. The late phase of mRNA accumulation (beyond 3 h after partial hepatectomy) is typical of liver regeneration. The level of c-myc mRNA is transiently increased (20-fold over normal) 20 h after partial hepatectomy, that is, at the time of DNA synthesis. Topoisomerase-I mRNA level increases between 3 and 24 h after partial hepatectomy (5 -10-fold over normal). These results suggest that accumulation of c-myc and topoisomerase-I mRNAs is associated with DNA replication in regenerating liver.Sequential activation of growth-related genes has been described in a number of proliferating cells, especially in mitogen-stimulated fibroblasts [l] and lymphocytes [2]. These genes probably characterize the sequential steps in the GO/Gl transition.Regenerating liver provides normal, in-vivo-proliferating hepatocytes. Although this system is complex and is under the control of a number of mitogenic stimuli [3,4], the sequence of mRNA induction in regenerating liver follows the same kinetics as in cultured fibroblasts: a transient increase in the level of the c$os mRNA (30 min after partial hepatectomy) [5] is followed by an increase in c-myc mRNA (3 h after partial hepatectomy) [6] and Ha-ras mRNA (24 h after partial hepatectomy) [7]. The accumulation of c--0.7 mRNA has also been reproduced in cultures of primary rat hepatocytes stimulated by EGF [5]. Other genes are activated in regenerating rat liver during the progression towards the S phase; namely the genes coding for actin and tubulin (8 h after partial hepatectomy) [8], p53 (8 h after partial hepatectomy) [9] and Kiras (> 12 h after partial hepatectomy) [lo].In a previous work, we have found that a and P-fibrinogen mRNAs were accumulated beyond 8 h after partial hepatectomy [l 11. Here, we describe that a transient increase in the level of fibrinogen mRNAs is detectable as soon as 15 min after partial hepatectomy. In addition, the same feature is shared by the actin mRNA, the level of which is also increased 15 min after partial hepatectomy. Moreover, this early activation was also found to take place after a sham operation. This result raised two questions : (a) Are proliferation-specific mRNAs also accumulated after a sham operation or, more generally, after stress? (b) Is it possible to distinguish between the events which are specific for cell proliferation and those which are related to stress? We have tried to answer these questions by studying, in the course of liver regeneration, the accumulation of ...

An enzyme which catalyzes the transamination of Caminobutyrate with 2-oxoglutarate was purified 588-fold to homogeneity from Candida guilliermondii var. membranuefaciens, grown with 4-aminobutyrate as sole source of nitrogen.An apparent relative molecular mass of 107000 was estimated by gel filtration. The enzyme was found to be a dimer made up of two subunits identical in molecular mass ( M , 55000).The enzyme has a maximum activity in the pH range 7.8 -8.0 and a temperature optimum of 45 "C. 2-Oxoglutarate protects the enzyme from heat inactivation better than pyridoxal 5'-phosphate. The absorption spectrum of the enzyme exhibits two maxima at 412 nm and 330 nm. The purified enzyme catalyzes the transamination of o-amino acids; 4-aminobutyrate is the best amino donorThe Michaelis constants are 1.5 mM for 2-oxoglutarate and 2.3 mM for 4-aminobutyrate. Several amino acids, such as a,/?-alanine and 2-aminobutyrate, are inhibitors (Ki = 38.7 mM, Ki = 35.5 mM and low activity is observed with B-alanine.and Ki = 33.2 mM respectively). Propionic and butyric acids are also inhibitors (Ki = 3 mM and Ki = 2 mM).Considerable efforts have been devoted during the recent years to characterize 4-aminobutyrate transaminase (GABA-T), a key enzyme involved in the metabolism of 4-aminobutyrate.Most of these studies have been carried out using extracts of various mammalian tissues [l -41, while a very few were undertaken on bacterial [5 -71 or yeast [8,9] homogenates.GABA-T has received attention because its substrate acts as an inhibitory transmitter, and therefore its selective inhibition is pharmacologically interesting for the physiological effect produced by a deliberate decrease of its activity after specific chemical modifications in vivo [lo-121.

No abstract