Chemistry, occurrence and biological effects of saturated pyrrolizidine alkaloids associated with endophyte-grass interactions

Abstract

Saturated amino pyrrolizidine alkaloids with an oxygen bridge between C-2 and C-7 have been isolated from grasses and the chemistry and biology of these substances are reviewed in this paper. These substances are commonly referred to as loline alkaloids as they are derivatives of loline and originally were isolated from Lolium spp. The saturated amino pyrrolizidine alkaloids are not hepatotoxic and must be distinguished from the 1,2-unsaturated pyrrolizidine alkaloids which are very significant animal and human toxins and carcinogens of plant origin. Chemical synthesis of loline has been achieved based on starting materials of a nitrone and methyl 4-hydroxycrotonate. N-formylloline is made by addition of ethyl formate to loline and removal of excess ethyl formate and ethanol formed. N-acetylloline is synthesized by addition of acetyl chloride to loline in chlorofirm. N-acetylloline is partitioned into a dilute acid, then from a dilute base with CHCl 3, and the CHCl 3 removed to yield N-acetylloline. N-methylloline, norloline, N-formynorloline, N-acetylloline, and N-propionylnorloline are also readily formed from loline. Little is known about the biosynthesis of the loline alkaloids but from biosynthesis of other pyrrolizidine alkaloids a proposed biosynthesis is ornithine → putrescine → spermidine → → dialdehyde → → norloline → → loline. Loline was first isolated from Lolium temulentum L. ( Lolium cuneatum Nevski) and later several derivatives were identified in tall fescue ( Festuca arundinacea Schreb.). Chromatographic methodologies have been used to separate the pyrrolizidine alkaloids but 50-m capillary columns (0.32 mm i.d.) or 10-m wide-bore columns (0.53 mm i.d.) provide the greatest resolution of the many loline derivatives. Loline alkaloids have been found primarily in plants known to be infected witn Acremonium coenophialum Morgan-Jones and Gams. Lolines have been found in Lolium perenne L., F. arundinacea, and Poa autumnalus Muhl. ex Ell. infected with A. coenophialum and in Festuca gigantea (L.) Vill. infected with an unidentified Acremonium spp. Loline alkaloids were present in infected tall fescue in greatest amounts in seed, followed in decreasing amounts in the rachis, stem, leaf sheath, and leaf blade. Site of synthesis, plant and/or endophyte, is not known nor is the translocation tissue known. During the main part of the growing season there is little change in the accumulation of the loline alkaloids in leaf blade. Alkaloid levels increased with plant age and increased in regrowth tissue. The concentration of endophyte mycelium in leaf sheaths is positively associated with loline alkaloid accumulation. N-formylloline is toxic to several different insects from ingestion, topical and injected applications. Larger animal bioassay have not been conducted with pure compounds but the observations suggest some pharmacological activity in these systems.