King-Armstrong unit

An obsolete unit used to assay the enzyme phosphatase, particularly in blood serum. One King-Armstrong unit was originally the amount of phosphatase that, acting upon disodium phenylphosphate in excess for 30 minutes at 37.5°C, at pH 9.0, liberates 1 milligram of phenol. Sometimes simply called a “King”. It was designed to yield numerical values equal to Jenner and Kay units.

Over the years various modifications to the method were proposed. For example, King's last version uses an incubation time of 15 minutes instead of 30², but reportedly the various modifications made little change in the value of the unit. A brief history of modifications, including directions for its last version (King and Wooton, 1956) is provided by Moss et al³.

1. Earl J. King and A. Riley Armstrong.
A convenient method for determining serum and bile phosphatase activity.
Canadian Medical Association Journal, vol. 34, pages 376-381 (1934).

2. E. J. King and I. D. P. Wooten.
Microanalysis in Medical Biochemistry. 3rd ed.
London: Churchill, 1956.

3. D. W. Moss, D. N. Baron, P. G. Walker and J. H. Wilkinson.
Standardization of clinical enzyme assays.
Journal of Clinical Pathology, vol. 24, pages 740-743 (1971).

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