The effects of a pleiotropic mutation ssd are described. This mutation results in decreased efficiency in the use of glucose and fructose as carbon source, inability to use succinate or to grow anaerobically, an alteration in the activity of enzymes responsible for the synthesis and degradation of L-serine, increased resistance to certain antibiotics, and a deficiency in proline transport. This mutation resembles various previously described mutations thought to affect' energy coupling factor' and is located in the same region of the chromosome. While the gene product affected by this mutation is still unidentified, it is clear that L-serine metabolism cannot be understood merely in terms of providing L-serine and its derivatives.

The changes in phospholipid composition sf the inner (cytoplasmic) surface of the human erythrocyte membrane resulting from the digestion of sealed inside-out vesicles with phospholipases A, and C were determined. Virtually all of the phosphatidylethanolamine and phosphatidylserine and 30-48%; of the phosphatidylcholine and sphingomyelin sf insideout vesicles were found to be accessible to enzyme hydrolysis. In contrast, all of the above phospholipids of unsealed ghosts were susceptible to phospholipolytic digestion. These results are a direct demoi~stration of an asymmetric distribution of phospholipids in the human erythrocyte membrane. KahHenberg, A., Walker, C. & Rohrlick, W. (1974) Evidence for an Asymmetric Distribution of PhosphoIipids in the Human Erythrocyte Membrane. Can. J. Biochem. 52, 803-886 Wous avons dCterminC les changements apportks dans la composition ghospholipidique B la surface intkrieure (cytoplasmiqile) de la membrane des Crytkrscytes humains suite k H a digestion par les phsspholipases As et C de vksicules morp-pkslogiquemerat inverskes et scellbes, Preque toutes les phssphatidylCthano1amines et les phosphatidylsCrines et 30-40CjI des phosphatidylcholiraes et des sphingomy6Enes des vCsisules inverskes sont touchCes par H'hydrolyse enzymatique. Au contraire, tous ces phospholigides sont susceptibles de digestion ghssgholipolytique dam les membranes strsmatiques non scelldes. Ces rCsuHtats dbmontrent direetement la distribution asymdtrique des phospholipides dans la membrane Crythrocytaire hurnaine. [Traduit par le jounral]lntrgeductim ghosts have provided evidence for an asym-~h~ action of group-specific labels and metrical distribution of membrane phospholipids phospholipases (4, 5 ) towards erythrocytes and (see Ref. 6). In contrast to cell ghosts9 the -

L-Serine deaminase is inactive in crude extracts of Escherichia coli K12, but can be activated by incubation with iron and dithiothreitol. This activation requires oxygen, and is inhibited by free radical scavengers and by diethylene triamine pentaacetic acid, which prevents Fe cycling. We suggest that in vitro activation of L-serine deaminase is catalyzed by an oxidant (perhaps hydroxyl radicals). Also, activation may be accompanied by a decrease in molecular weight and involve both a cleavage of the polypeptide chain and a reversible reduction of the molecule.

Escherichia coli L-serine deaminase (L-SD) in crude extracts made in glycylglycine could be activated by incubation with iron sulfate and dithiothreitol. This activation could also be demonstrated in vitro in two mutants which were physiologically deficient in L-SD activity in vivo. This suggests that these mutants were deficient not in L-SD but in an enzyme(s) activating L-SD. The suggestion is made that production of a functional L-SD in vivo requires activation of the structural gene product by an enzyme or enzymes that reduce the protein to an active form.

Two methods for the direct isolation of spontaneous ssd mutants of Escherichia coli K-12 strains are described; (i) by growth with L-serine as the carbon source, and (ii) by low-level kanamycin resistance. A newly isolated mutant had the same phenotype as the mutant described previously, including inefficient use of glucose, inability to grow with succinate, altered transport characteristics, and altered resistance to certain growth effectors. Succinate-utilizing derivatives which appear to be intragenic are characterized in detail. The relation between the mutants isolated here and mutants which are thought to have impairments in a system of coupling respiratory energy to active transport (eciB mutants) is discussed.