Acetylcholinesterase (EC 3.1.1.7.; AChE) and butyrylcholinesterase (EC 3.1.1.8.; BuChE) from chicken muscle exist as sets of structurally homologous forms with very similar properties. The collagenase sensitivity and aggregation properties of the 'heavy' forms of both enzymes indicate that they possess a collagen-like tail, and their stepwise dissociation by trypsin confirms that they correspond to triple (A12) and double (A8) collagen-tailed tetramers. In addition to this dissociating effect, trypsin digests an important fraction of the catalytic units of AChE, in a progressive manner, removing as much as 30% of the enzyme's mass, without inactivation of the tetramers and of the tailed molecules. The trypsin-modified AChE forms closely resemble the corresponding mammalian AChE forms in their hydrodynamic properties. It is not known whether the trypsin-digestible peptides, which do not appear to be involved in the ionic or hydrophobic interactions of the enzymes, are a fragment of the catalytic subunit or whether they constitute distinct polypeptides.

~Comparison of the pools of glutamic acid and glutamine and of the specific activities of glutamine synthetase and glutamate dehydrogenases in sporulating a/cc and non-sporulating a/cc cells of Saccharomyces cerevisiae revealed a difference in their nitrogen metabolism.Glutamhe synthetase and glutamine appeared to be necessary for the sporulation process, glutamine playing, at least, a catabolic role. However, exogenous glutamine as well as ammoniainhibited sporulationwhile glutamic acid did not. Glutamine seemed to act through its amino group. Both inhibitors had at least two sites of action, one effective early in sporulation and related to DNA synthesis and the other acting later and not related to it. I N T R O D U C T I O NStarvation of the yeast Saccharomyces eerevisiae induces a differentiation process leading to meiosis and sporulation of diploid ( a / a ) cells. This process is inhibited by ammonia and some other nitrogen compounds (Miller, 1963; Piiion, 1977) but very little is known about the metabolism of ammonia in sporulating cells. In this paper, we report our studies on the activities of enzymes involved in ammonia assimilation (glutamhe synthetase, NAD-and NADP-linked glutamate dehydrogenases), the effect of the substrates of these enzymes on the sporulation process, and the variation in the sizes of pools of free glutamine and glutsmic acid during the first hours after the transfer of cells to the sporulation medium with or without ammonia. METHODS Yeast strains, growth and sporulation. Diploid cells of Saccharomyces cerevisiae 532 a / a and 533 ala,isogenic except for their mating type (Pellucuer, 19731, were used. The a / a cells were grown and allowed to sporulate as described by Durieu-Trautmann & Delavier-Klutchko (1977). The alee cells, which are unable to sporulate, were used as a control.Enzyme activities. Extracts were prepared at 4 "C by passing suspensions of cells in 100 mwpotassium phosphate buffer pH 7-8, containing 1 mhl-2-mercaptoethano1, through a French press at 100 MPa. Cell debris was removed by centrifuging for 30 min at 30000 g and the supernatant was dialysed overnight at 4 "C against buffer similar to that used for extraction.Activities of NAD-linked glutamate dehydrogenase (NAD-Gdh; EC 1 .4.1.2) and of NADP-linked glutamate dehydrogenase (NADP-Gdh; EC 1.4.1.4) were estimated by determining the rate of NADH or NADPH oxidation at 25 "C (Boehringer, 1968). The rate of reaction with NAD-Gdh was corrected by subtracting the rate of NADH oxidation observed in the absence of 2-oxoglutarate.The activity of glutamine synthetase (GS; EC 6.3.1 .2) was measured by assaying transferase activity (Woolfolk et al., 1966).Protein was determined by the Folin method of Lowry, using bovine serum albumin as standard.

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