Mitochondria from various organisms, especially plants, fungi and many bact
eria contain so-called alternative NADH:ubiquinone oxidoreductases that cat
alyse the same redox reaction as respiratory chain complex I, but do not co
ntribute to the generation of transmembrane proton gradients. In eucaryotes
, these enzymes are associated with the mitochondrial inner membrane, with
their NADH reaction site facing either the mitochondrial matrix (internal a
lternative NADH:ubiquinone oxidoreductases) or the cytoplasm (external alte
rnative NADH:ubiquinone oxidoreductases). Some of these enzymes also accept
NADPH as substrate, some require calcium for activity. In the past few yea
rs, the characterisation of several alternative NADH:ubiquinone oxidoreduct
ases on the DNA and on the protein level, of substrate specificities, mitoc
hondrial import and targeting to the mitochondrial inner membrane has great
ly improved our understanding of these enzymes. The present review will, wi
th an emphasis on yeast model systems, illuminate various aspects of the bi
ochemistry of alternative NADH:ubiquinone oxidoreductases, address recent d
evelopments and discuss some of the questions still open in the field. (C)
2000 Elsevier Science B.V. All rights reserved.