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these early investigations, but the then available analytical armament was
insufficient for an effective tackling of these complex plant constituents.
Interest in this field waned therefore considerably, and it was not before
the 1950s when, in response to significant progress in more effective
separation techniques and sensitive analytical procedures, a remarkable
renaissance could be observed with the seminal investigations of O. Th.
Schmidt and W. Mayer (for a competent review of tannin research from
the beginnings to the mid-1950s, see Schmidt and Mayer, 1956). This
trend was continued by other research groups, mainly in England and
Japan, including, among many others, the laboratories of E. C. Bate-
Smith, T. Swain, E. Haslam, T. Okuda and I. Nishioka (see Chapters 1
and 2). Detailed structures of a myriad of hydrolyzable tannins and
related compounds were compiled by these efforts ( e.g. , Haslam, 1989)
that not only served to describe their distribution and evolutionary
relationships in the plant kingdom ( e.g. , Haddock et al. , 1982, Okuda et
al. , 1993), but also provoked considerations on possible routes in their
biogenetic origin (Haddock et al. , 1982), an aspect, however, that was
merely hypothetical at these times.
Concerning this latter question, it must be emphasized that
biosynthetic studies were initially based on so-called 'feeding'
experiments, a technique by which radioactively labeled putative
precursors were administered to living plants or plant parts. Next,
investigated compounds were isolated; total amount and distribution of
radioactivity in these products served as a basis for the construction of
feasible metabolic routes. The introduction of enzyme studies added a
new dimension to such investigations, hence allowing experiments with
labile or membrane-impermeable compounds and, in particular, giving
access to work with “energy-rich” activated intermediates. Over the past
25 years, such enzyme studies on the challenging question of
hydrolyzable tannin biosynthesis were performed in my laboratory. In
retrospect, it was a rather daring endeavor to tackle this problem just by
enzymological means if one considers the “tanning” property, i.e. ,
protein precipitating, and hence enzyme inactivating potential of such
compounds. Fortunately, gallotannin- and ellagitannin-synthesizing
enzymes proved to be remarkably resistant to their unfavorable
substrates and products. In contrast, major problems were encountered in
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