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J S Krouwer

Publications and source records attributed to J S Krouwer.

6 recordsLinked to original sources
PubMed1992

Estimating total analytical error and its sources. Techniques to improve method evaluation.

The process of method evaluation starts with identifying goals either to demonstrate the clinical validity of an assay or to identify assay error sources that require improvement. Taguchi's idea of continual quality improvement vs the notion of meeting or failing specification has been applied to clinical chemistry. In this article, I propose a model of assay performance that includes the terms random interferences and protocol-specific biases (a series of systematic errors). I explain these terms, as well as the consequences of failing to consider them. To validate an assay clinically, I recommend direct estimation of total analytical error from a method comparison. To identify assay error sources that require improvement, I recommend a multifactor protocol (in addition to a method comparison). Individual error sources are related to total analytical error with the use of an error propagation technique. Much of the proposed data analysis techniques are straightforward but not routinely practiced. I demonstrate principles with the use of a cholesterol assay.

Chemistry, Clinical↗
PubMed1991

Multi-factor designs. IV. How multi-factor designs improve the estimate of total error by accounting for protocol-specific biases.

Total error is often calculated as a combination of random error and fixed bias. However, the specific protocols used to estimate random error and fixed bias are themselves variable factors that can affect the estimate of total error. We refer to biases such as assay drift, sample-to-sample carryover, and reagent carryover as examples of fixed biases that are protocol-specific and distinguish them from other fixed biases. Failing to account for protocol-specific biases that are present will lead to incorrect estimates of total error when routine use of the assay involves a protocol different from that used to estimate total error. Multi-factor protocols are recommended to determine protocol-specific biases, which, if present, should be included in the estimate of total error.

Carcinoembryonic Antigen↗
PubMed1978

The mechanism of action of ethanolamine ammonia-lyase, an adenosylcobalamin-dependent enzyme. Reaction of the enzyme.cofactor complex with 2-aminoacetaldehyde.

Ethanolamine ammonia-lyase (EC 4.3.1.7) catalyzes the adenosylcobalamin-dependent deamination of ethanolamine and 2-aminopropanol. Incubation of the enzyme.cofactor complex with 2-aminoacetaldehyde leads to rapid cleavage of the carbon--cobalt bond accompanied by the destruction of the corrinoid portion of the cofactor. During this reaction the adenosyl portion of the cofactor is oxidized to 4',5'-anhydroadenosine, and the aminoacetaldehyde is converted to acetic acid, which remains associated with the enzyme as a noncovalent complex which survives gel filtration. There is no evidence for the alkylation of the corrin metal by the substrate analog. The enzyme.AdoCbl complex is thus able to eliminate an amino group from a substrate analog without the formation of a new alkyl cobalamin in which the analog is a ligand. These observations do not support the participation of what might be termed "substratylcobalamin" as an intermediate in the ammonia migration occurring in reactions catalyzed by ethanolamine ammonia-lyase.

Ammonia-Lyases↗
PubMed1977

The mechanism of cobalamin-dependent rearrangements.

Adenosylcobalamin-dependent rearrangements are enzyme catalyzed reactions in which a hydrogen atom is transfered from one carbon atom to an adjacent one in exchange for a group X which migrates in the opposite direction. In the hydrogen transfer step, the mechanism of which is reasonably well understood, the cofactor serves as an intermediate hydrogen carrier. The transfer of hydrogen to the cofactor involves homolysis of the carbon-cobalt bond to generate cob(II) alamin and the 5'-deoxyadenos-5'-yl radical, followed by abstraction of a hydrogen atom from the substrate to form 5'-deoxyadenosine and the substrate radical. After migration of group X, the hydrogen atom is returned to the product radical by the reverse of the above reactions to generate the final product and reconstitute the cofactor. In contrast to the transfer of hydrogen, the mechanism of group X migration is poorly understood. Many reactions mechanisms have been proposed on chemical grounds, but there is insufficient biochemical evidence to permit a choice among these propsals. A quantity of negative evidence has accumulated suggesting that group X migration does not involve alkylation of the cobalt of cobalamin by the substrate, but in the absence of firm data supporting an alternative mechanism, even this weak conclusion must be regarded as provisional.

Chemical Phenomena↗