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The physicochemical properties of eight extracellular proteinases secreted by psychrotrophic bacteria of dairy origin have been studied. Seven of these proteinases were able to withstand ultra heat treatment (UHT) with D values at 140 °C ranging from 2 to 300 s. The six Pseudomonas fluorescens proteinases were glycoproteins of mol. wt 47000-49500. The two Serratia marcescens proteinases, of mol. wt of 51000, did not contain carbohydrate but in other respects were similar to the Pseudomonas proteinases. The proteinases were inhibited by various metal chelators and all contained Ca and Zn in similar proportions. Their amino acid compositions were similar, with alanine as the N-terminal group, cysteine completely absent and very low levels of methionine. Isoelectric points ranged from 5-10 to 8-25. Their physical and chemical properties enabled them to be classified as alkaline metalloendopeptidases. A similarity index (SAn) was used to predict sequence homology between ten proteinases of known amino acid composition. Comparisons of SAn of these proteinases showed only minor sequence differences except for those of Ps. fluorescens MC60. Heat resistance could not be related wholly to similarities in protein sequence, but could be related both to the strength of stabilizing Ca 2+ -protein interactions and to the randomness inherent within the folding of the peptide chain.

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The isolation by affinity chromatography of a low‐molecular‐weight form of phosphofructokinase from Escherichia coli K‐12 has resulted in an improved 2050‐fold purification. The enzyme was readily separable from the higher‐molecular‐weight form by Sepharose 6B fractionation. The enzyme was found to have a molecular weight of 65000 ± 6500 with a subunit molecular weight of 35000 ± 3500. The enzyme exists in either of two interconvertible forms dependent on the presence or absence of the positive of ectors ADP, fructose 6‐phosphate and ATP. A regulatory mechanism is postulated for this enzyme involving hysteric conformational changes; its importance in the mechanism of the Pasteur effect is discussed.